To mix this batter by hand, put ingredients into a large bowl. Using a wooden spoon, stir them together until just combined, scraping down the sides of the bowl as needed.
- 3/4 cup (1 1/2 sticks) unsalted butter, softened
- 1 1/2 cups granulated sugar
- 6 large egg whites
- 3/4 cup whole milk
- 2 teaspoons pure vanilla extract
- 2 vanilla beans, halved lengthwise, seeds scraped and reserved, beans discarded
- 2 cups all-purpose flour
- 2 teaspoons baking powder
- 1/4 teaspoon coarse salt
- 4 ounces bittersweet chocolate, cut into 12 2-inch chunks
- 1 tablespoon confectioners' sugar, for dusting
Preheat oven to 350 degrees. Line a standard 12-cup muffin tin with paper liners; set aside. Put butter and granulated sugar into the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy, about 3 minutes. Add egg whites, milk, vanilla extract, and vanilla seeds; mix until combined, scraping down sides of bowl as needed. Add flour, baking powder, and salt; mix until just combined.
Divide batter among muffin cups, filling each cup about 3/4 full. Insert a piece of chocolate into center of each, pressing about halfway into batter to cover. Bake until pale golden brown, about 24 minutes. Transfer tin to a wire rack; let cool slightly. Dust with confectioners' sugar before serving.