Chocolate-Filled Cupcakes

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. Cupcakes can be stored in airtight containers at room temperature up to 3 days.