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1
Position oven rack in the center of the oven; preheat the oven to 350; butter and flour two 9 x2 inch round cake pans.
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2
Make the cake--Whisk together the cocoa and boiling water in a small bowl until smooth.
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3
Whisk the flour, baking soda, and salt in a medium bowl.
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4
Beat the butter, sugar, and vanilla with an electric mixer on medium speed in a large deep bowl until well blended; the mixture will be dry.
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5
Add in the eggs one at a time, beating well after each addition.
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6
Decrease speed to low and beat in the flour mixture alternately with the sour cream, beginning and ending with the flour, just until well blended.
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7
Add the cocoa mixture and blend well.
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8
Divide the batter evenly between the prepared pans.
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9
Bake for 40 minutes, or until a pick comes out clean; cool the cakes in the pans on wire racks for 20 minutes, then invert cakes onto the racks and remove the pans.
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10
Turn the cakes right-side up and cool completely.
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11
Make the frosting--melt the chocolate with the water in a small heatproof bowl set over a saucepan of about 1 1/2 inches of nearly simmering water, whisking until smooth; transfer to a large deep bowl.
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12
Heat the sugar with the egg yolks in another heatproof bowl over the same saucepan of nearly simmering water, whisking constantly, for about 4 minutes, until the sugar is dissolved and the mixture is warm to touch.
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13
Whisk the sugar mixture into the chocolate mixture, add in the vanilla, and beat with an electric mixer on high speed for 3 minutes, or until the mixture has cooled to room temperature.
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14
Beat in the butter one piece at a time, beating until smooth after each addition.
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15
Assemble the cake--Place 1 cake layer on a serving plate; spread about 1 cup of frosting evenly over the layer.
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16
Top with the remaining cake layer and frost with the remaining frosting; serve cut into wedges.
