Triple Chocolate Cupcakes

For the cupcakes:

unsweetened Dutch-process cocoa powder

hot water

all-purpose flour

baking soda

baking powder

coarse salt

unsalted butter

sugar

large eggs, at room temperature

vanilla extract

sour cream, at room temperature

For the ganache filling:

bittersweet chocolate, finely chopped

plus 2 tbsp. heavy cream

unsalted butter, cut into 4 pieces, at room temperature

For the frosting:

bittersweet chocolate, finely chopped

cream cheese, at room temperature

unsalted butter, at room temperature

confectioners’ sugar, sifted

unsweetened Dutch-process cocoa powder

salt

plus 2 tbsp. sour cream

For the chocolate curls:

Large block of high-quality semi- or bittersweet chocolate