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1
Preheat the oven to 350 degrees F.
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2
Spray two 8-inch cake pans with 1 1/2-inch sides lightly with cooking spray.
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3
Arrange racks in the middle of the oven.
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4
Beat the eggs, butter, water, and cocoa powder together in a large bowl in a standing mixer until smooth.
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5
Whisk the flour, sugar, baking powder, baking soda, and salt together in a second bowl.
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6
Add the dry mixture to the wet ingredients, and beat until evenly blended and smooth, 2 to 3 minutes, scraping down the sides with a spatula.
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7
Divide the batter between the two greased pans.
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8
Bake for 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean.
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9
Cool in pans for 5 minutes before turning out onto wire cooling rack.
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10
Cool completely before frosting.
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11
For the frosting: Heat the butter and chocolate in a heavy-bottomed saucepan over moderate heat.
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12
Stir constantly with a rubber spatula until the mixture is smooth and melted.
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13
Remove the saucepan from the heat, and whisk in the cocoa powder and vanilla until the mixture is smooth and the cocoa is dissolved.
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14
Beat in confectioners sugar and sour cream with a hand mixer, beating until smooth and creamy.
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15
To assemble the cake: Place one layer, top side down, on a cake stand.
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16
Spread on it a thick layer of frosting with a spatula.
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17
Place the next layer of cake right side up on top of the frosting.
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18
Frost the top and sides of the cake by working from the center of the cake toward the edge, making sure to coat evenly.
