Preheat and prepare: Set oven to 350°F. Line a standard 12-cup muffin tin with paper or silicone cupcake liners. This ensures the cupcakes won't stick and creates a neat presentation. Checkpoint: You're ready to move forward when the liners are secure in every cup.

Mix dry ingredients: In a small bowl, whisk together 1 3/4 cups all-purpose flour, 1 1/2 teaspoons baking powder, and 1/4 teaspoon salt. Set aside. This combination aerates and seasons the flour evenly. Checkpoint: The mixture should look uniform with no lumps.

Cream butter and sugar: In a large bowl using an electric mixer, beat 1/2 cup softened unsalted butter with 1 cup granulated sugar on medium-high speed for 2–3 minutes. The mixture should look pale, fluffy, and increased in volume by about one-third. This creaming process incorporates air, giving cupcakes a tender crumb. Checkpoint: The mixture should be light in color and fluffy—almost mousse-like.

Add eggs one at a time: Add 2 large eggs one at a time, beating on medium speed for 30–45 seconds after each addition. Scrape down the sides of the bowl between eggs. Adding eggs gradually prevents the batter from curdling. Checkpoint: The batter should look smooth and creamy after each egg.

Add vanilla: Beat in 1 teaspoon vanilla extract for 15–20 seconds on medium speed until fully combined. Checkpoint: Vanilla should be incorporated with no streaks visible.

Alternate dry and wet ingredients: Add one-third of the flour mixture to the butter mixture, beat on low speed for 20–30 seconds just until combined (don't overmix). Then add half of the 1/2 cup sour cream, beat gently for 20 seconds. Repeat: one-third flour, remaining sour cream, remaining flour. Overmixing develops gluten, making cupcakes tough, so mix only until you see no streaks of flour. Checkpoint: The batter should be smooth, thick, and fall easily from a spoon.

Add lemon juice and zest: Gently fold in 1/4 cup fresh lemon juice and 2 tablespoons lemon zest using a rubber spatula with a few broad sweeping motions—about 10–15 strokes total. Don't fully incorporate; a few streaks are okay. This preserves the tender crumb. Checkpoint: You should see flecks of lemon zest throughout the batter.

Fill liners: Divide the batter evenly among 12 cupcake liners, filling each about two-thirds full. Use an ice cream scoop (about 2 tablespoons per cupcake) for consistent sizing and even baking. Checkpoint: All liners should be equally filled with no empty cups.

Bake cupcakes: Place the muffin tin in the preheated 350°F oven on the center rack. Bake for 18–22 minutes until a wooden toothpick inserted in the center of a middle cupcake comes out with just a few moist crumbs (not wet, but not completely dry). The tops should be golden brown and spring back lightly when touched. Checkpoint: The toothpick test is your guide—don't overbake or cupcakes will be dry.

Cool in pan: Remove the muffin tin from the oven and let cupcakes rest in the pan for exactly 5 minutes. This allows them to set slightly so they don't fall apart when removed. Don't skip this step. Checkpoint: Cupcakes should feel firm to the touch but still warm.

Transfer to cooling rack: Gently turn the muffin tin on its side and carefully lift the cupcakes out onto a wire rack to cool for at least 1 hour until completely cool to the touch. Never frost warm cupcakes or the frosting will slide off. If you're in a warm kitchen, place the cooling rack in front of a fan or in a cooler room to speed cooling. Checkpoint: Cupcakes should be at room temperature with no warmth when you touch the sides.

Make lemon curd filling: In a small saucepan, whisk together 1/2 cup sweetened condensed milk and 2 tablespoons cornstarch until smooth—no lumps. Add 3 tablespoons fresh lemon juice and 1 tablespoon lemon zest. Checkpoint: The mixture should be smooth and pale yellow.

Cook lemon curd: Place the saucepan over medium heat (not high or the curd will curdle). Stir constantly with a rubber spatula, scraping the bottom and sides. Cook for 4–6 minutes until the mixture thickens noticeably—it should coat the back of a spoon and leave a clear trail when you run your finger through it. The mixture will continue to thicken as it cools. Remove from heat immediately; don't let it boil. Checkpoint: The curd should be glossy, thick, and pale yellow-gold in color.

Cool lemon curd: Let the curd rest at room temperature for 10–15 minutes, stirring occasionally. Once slightly cooled, transfer to a piping bag fitted with a small round tip (or use a small spoon). Set aside. Checkpoint: The curd should be cool enough to handle but still spreadable.

Make lemon frosting: In a medium bowl using an electric mixer, beat 1/2 cup softened unsalted butter on medium speed for 1 minute until creamy and pale. Checkpoint: Butter should look light and fluffy.

Add powdered sugar: Gradually add 2 cups powdered sugar (sift it if lumpy) to the butter, 1/2 cup at a time, beating on medium speed for 30–45 seconds after each addition. Scrape down the sides of the bowl between additions. Checkpoint: The mixture should be fluffy, light, and pale yellow after all sugar is incorporated.

Add lemon juice and zest to frosting: Add 3 tablespoons fresh lemon juice and 1 tablespoon lemon zest, beating on medium speed for 30–45 seconds until fully combined and fluffy. The frosting should be spreadable and airy, not runny. Checkpoint: Frosting should hold stiff peaks when you lift the mixer beater.

Add salt to frosting: Beat in a small pinch of salt (about 1/8 teaspoon) for 10–15 seconds. Salt enhances the lemon flavor and balances sweetness. Checkpoint: No grittiness should be visible; frosting should be smooth and vibrant yellow.

Core the cupcakes: Once cupcakes are completely cool, use a small paring knife or a cupcake corer (a small cylindrical tool) to carefully remove a small cone or circle from the top center of each cupcake, about 1 inch deep and 3/4 inch in diameter. Be careful not to cut all the way through the bottom. You should have a small cavity to fill. Checkpoint: Each cupcake should have a clean, shallow cavity without cracks.

Fill with lemon curd: Using the piping bag or a small spoon, carefully fill the cavity in each cupcake with the lemon curd filling. Fill until just below the rim—don't overfill or it will ooze out when frosted. Checkpoint: Each cupcake should have visible yellow filling in its center.

Frost the cupcakes: Transfer the lemon frosting to a piping bag fitted with a decorative tip (star tip recommended for visual appeal) or use an offset spatula for a rustic look. Pipe or spread a generous dome of frosting on top of each cupcake, covering the filled center. Use an ice cream scoop (about 2 tablespoons frosting per cupcake) to ensure consistency. Checkpoint: Each cupcake should be completely covered with frosting with no cake visible.

Garnish: While frosting is still slightly soft, top each cupcake with a sprinkle of fresh lemon zest or a very thin lemon wheel slice. This adds visual appeal and a pop of fresh citrus flavor. Checkpoint: All cupcakes should be garnished uniformly.

Final chill (optional but recommended): Refrigerate the finished cupcakes for 15–20 minutes before serving. This allows the frosting to firm up and makes them easier to handle and eat. The cupcakes will taste even better when slightly chilled. Checkpoint: Frosting should feel firm to the touch but still yield slightly when pressed.