Best Chocolate Cake

INGREDIENTS

8 oz

bittersweet chocolate, finely chopped

2/3 cup

Dutch-process cocoa, sifted

2 tsp

instant espresso powder*

1 cup

boiling water

1 3/4 cups

all-purpose flour

1 cup

granulated sugar

1/2 cup

packed light brown sugar

1 tsp

salt

1 tsp

baking soda

1 cup

sour cream, at room temperature

1/2 cup

vegetable oil*

large eggs plus one egg yolk, at room temperature

1 tbsp

white vinegar

2 tsp

vanilla extract

2/3 cup

granulated sugar

large egg whites

1/4 tsp

salt

1 1/2 cups

unsalted butter, cut into 12 pieces and at room temperature

12 oz

semisweet chocolate, melted and cooled slightly

1 tsp

vanilla extract