01 -
Beat the softened butter and powdered sugar together until pale and fluffy, about 2 minutes.
02 -
Mix in the egg yolk, vanilla extract, and cooled melted chocolate until smooth and glossy.
03 -
Whisk the flour, cornstarch, cocoa powder, baking powder, and salt in a separate bowl, then add to the wet mixture. Beat on low just until a soft dough forms.
04 -
Use a tablespoon to portion the dough into balls. Roll lightly and place 2 inches apart on a parchment-lined baking sheet. Flatten each ball slightly with the bottom of a glass.
05 -
Bake at 350 degrees F (175 degrees C) for 10 to 12 minutes until the tops look set and the edges are firm but not dark. The centers will still be soft.
06 -
Let the cookies rest on the pan for 5 minutes, then transfer to a wire rack. Once fully cool, dust generously with powdered sugar.