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1
For cake: heat oven to 350 degrees.
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2
Butter sides of two 8-inch cake pans, and line bottoms with parchment paper.
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3
In the bowl of a food processor fitted with a knife blade, combine flour, sugar, baking powder, baking soda, cocoa powder, butter, eggs, vanilla and sour cream.
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4
Process to make a smooth, thick batter.
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5
Using a rubber spatula, divide batter between pans, and smooth tops.
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6
Bake until a cake tester inserted in center comes out clean, 25 to 35 minutes; do not overbake.
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7
Transfer to a wire rack to cool for 10 minutes before removing from cake pans.
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8
For frosting: Combine chocolate and butter in a large heat-proof bowl, and heat until melted in a microwave oven or over a pan of simmering water.
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9
Remove from heat, and allow to cool for 5 minutes.
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10
Stir in corn syrup, sour cream, and vanilla.
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11
Whisk in confectioners' sugar until very smooth.
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12
Frosting should be thick and spreadable.
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13
If necessary, add a teaspoon or two of boiling water to thin it, or additional sifted confectioners' sugar to thicken.
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14
Cut four strips of waxed or parchment paper, and place them side by side on a cake plate, covering the surface.
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15
Place one cake layer domed-side down on plate.
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16
Spoon about a third of the frosting onto center of cake, and use a knife or a spatula to spread it evenly.
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17
Place the other cake on top, domed side up.
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18
Spoon another third of frosting on top of cake, spreading to make swirls or a smooth finish.
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19
Spread sides of cake with remaining frosting, and allow to sit for a few minutes until set.
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20
Carefully remove paper strips.
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21
Place cake under glass or in an airtight container, and set aside in a cool place until serving.
