Preheat your oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease each cup well with butter or cooking spray.

Chop the semisweet chocolate and place it in a microwave-safe bowl along with the baking chocolate squares and butter.

Microwave the chocolate mixture in 30-second intervals, stirring between each interval, until completely melted and smooth. This should take 1–2 minutes total. Do not overheat or the chocolate will scorch.

Let the melted chocolate cool to lukewarm (about 2–3 minutes). It should feel warm but not hot to the touch. Checkpoint: You are ready to move on when the chocolate is no longer steaming and feels just warm.

In a separate medium bowl, combine the sour cream, packed brown sugar, light corn syrup, egg, and vanilla extract.

Whisk the wet ingredients together for about 1 minute until smooth and well combined. The mixture should be slightly pale and thick.

Pour the sour cream mixture into the cooled chocolate and stir gently with a spatula until fully blended. The batter should look smooth and glossy with no white streaks of sour cream visible.

In a separate small bowl, whisk together the flour, baking soda, and salt. Make sure there are no lumps in the baking soda.

Add the dry ingredients to the chocolate mixture all at once and blend very well using a spatula or wooden spoon, stirring for about 1 minute until no flour streaks are visible. Do not overmix—just blend until combined.

Gently fold in the semi-sweet chocolate chips using a spatula. The batter should look thick, glossy, and studded with chocolate pieces.

Spoon the batter evenly into the prepared muffin cups, filling each about two-thirds full. Use an ice cream scoop or spoon to ensure even distribution. The batter should mound slightly above the rim of each cup.

Place the muffin tin on the middle oven rack and bake for about 20 minutes. The muffins are done when the tops are set but still slightly soft to the touch, and a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter, but a few chocolate smudges are perfect).

Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes. They will continue to cook slightly from residual heat.

Turn the muffins out of the tin onto a wire cooling rack and let them cool for at least 15–20 minutes before serving. Checkpoint: You are ready to eat or store when the muffins are cool to the touch.

If freezing, cool the muffins completely to room temperature first (about 30 minutes). Place each cooled muffin in an individual freezer bag, press out as much air as possible, seal tightly, and freeze for up to 3 months.