The Best Chocolate Cupcakes

INGREDIENTS

For the cupcakes::

4 oz

bittersweet chocolate, finely chopped

1/3 cup

Dutch-process cocoa

1 tsp

instant espresso powder

1/2 cup

boiling water

3/4 cup

plus 2 tablespoons (4 ounces) all-purpose flour

1/2 cup

granulated sugar

1/4 cup

packed light brown sugar

1/2 tsp

salt

1/2 tsp

baking soda

1/2 cup

sour cream, at room temperature (or plain full fat Greek yogurt)

1/4 cup

vegetable oil

large eggs plus one egg yolk, at room temperature

2 tsp

white vinegar

1 tsp

vanilla extract

For the frosting::

1/3 cup

granulated sugar

large egg whites

salt

12 tbsp

unsalted butter, cut into 12 pieces and at room temperature

6 oz

semisweet chocolate, melted and cooled slightly

1/2 tsp

vanilla extract