Preheat your oven to 350°F for 10 minutes. Line two baking sheets with parchment paper.

In a small bowl, whisk together 2 and 1/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon salt, and 1/2 teaspoon finely chopped fresh rosemary. Set aside. Checkpoint: Your dry ingredients should be evenly mixed with tiny specks of rosemary throughout.

In a large bowl using a hand mixer or standing mixer, beat 3/4 cup softened unsalted butter and 3/4 cup granulated sugar together on medium speed for 3 to 4 minutes until the mixture looks pale, fluffy, and light—like clouds. Scrape down the bowl occasionally. Checkpoint: You should see a noticeable increase in volume and the mixture should be lighter in color.

Add 1 large egg to the butter mixture. Beat on medium speed for 1 minute until fully combined and no streaks of egg remain.

Add 2 tablespoons fresh lemon juice, 1 tablespoon lemon zest, and 1 teaspoon vanilla extract. Beat on medium speed for 30 seconds until the filling is smooth and the lemon zest is evenly distributed. The mixture should smell strongly of lemon and butter.

With the mixer on low speed, add the flour mixture in three additions, mixing just until the dough comes together. Do not overmix or your cookies will be tough. Stop as soon as no dry flour streaks remain. Checkpoint: Your dough should be cohesive but still soft—not sticky.

Divide the dough in half. Place each half between two sheets of parchment paper. Using a rolling pin, gently roll each half to about 1/8 inch thick (thin, even thickness is key for uniform cookies). Refrigerate both rolled doughs for 20 minutes until firm and easy to cut.

Remove one sheet of dough from the refrigerator and carefully peel off the top parchment. Using a 1.5-inch round cookie cutter, cut out cookies by pressing straight down and lifting out. Place cookies 1 inch apart on prepared baking sheets. Repeat with the second sheet of dough. Gather dough scraps, re-roll, and cut additional cookies if desired.

Place both baking sheets in the preheated 350°F oven on the middle and upper-middle racks (rotate halfway through). Bake for 10 to 12 minutes until the edges of the cookies are barely golden and the centers still look slightly pale. The cookies will continue to set as they cool. Checkpoint: The edges should feel firm but the centers should still be tender when you gently press them. You should smell warm lemon and butter.

Remove baking sheets from the oven. Let cookies cool on the baking sheets for 5 minutes—they will firm up during this time. Using a spatula, carefully transfer cookies to a wire cooling rack. Let cool for at least 20 minutes at room temperature before filling. Checkpoint: Cookies should be completely cool and firm to the touch before you assemble them.

While cookies cool, prepare the filling. In a large bowl using a hand mixer or standing mixer, beat 1 cup softened unsalted butter on medium speed for 2 to 3 minutes until it is pale, creamy, and increased in volume.

Gradually add 2 cups powdered sugar while mixing on low speed. This prevents a cloud of sugar powder. Once the sugar is mostly combined, increase speed to medium and beat for 1 minute until the filling is light and fluffy. Scrape down the bowl occasionally.

Add 2 tablespoons fresh lemon juice, 1 tablespoon lemon zest, and 1/4 teaspoon finely minced fresh rosemary to the buttercream. Beat on low speed for 30 seconds, then on medium speed for 30 seconds more until everything is evenly incorporated. The filling should be pale yellow with visible specks of zest and rosemary. Checkpoint: Your filling should be spreadable but hold its shape—not runny or too stiff.

Transfer the filling to a piping bag fitted with a 1/2-inch round tip, or use a small spoon to portion. Match up cookie pairs by size and shape.

Holding one cookie flat side up, pipe or spread about 1 teaspoon of filling onto the flat side. Do not overfill or the filling will squish out the sides.

Press a second cookie flat side down onto the filling, gently pressing so the filling spreads to the edges. Repeat with all remaining cookies and filling.

Serve immediately or allow filled cookies to rest at room temperature for 30 minutes so the filling sets slightly. Checkpoint: The filling should feel set but still tender when you bite into it—not hard.

Troubleshooting bullet points:If your dough is too sticky to cut: refrigerate longer—cold dough is much easier to work with. You can also freeze rolled dough for up to 2 weeks and cut directly from frozen.

If your cookies spread too much during baking: your oven may be running hot. Lower the temperature by 10°F and check with an oven thermometer. Chill dough for 30 minutes before baking next time.

If your buttercream is too soft: refrigerate it for 10 minutes, then re-whip. Ensure all your butter was softened, not melted. Cold butter requires beating longer to become fluffy.

If your cookies taste too rosemary-forward: use only 1/4 teaspoon in the dough next time. Rosemary is potent; less is often better. You can always add more to the filling if desired.

If your filled cookies feel greasy: this means the filling has too much butter or your kitchen is very warm. Use the cookies within 1 hour or refrigerate. They taste best when the filling is cool and slightly set.