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1
Preheat the oven to 350.
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2
Butter two 9-by-2-inch round cake pans and line with parchment paper; butter and dust with flour.
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3
In a bowl, mix the 3 cups of flour, baking powder, and salt.
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4
In a cup, mix the milk with the vanilla.
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5
In a standing mixer fitted with the paddle, beat the butter at medium speed until light and creamy.
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6
Add the sugar and beat until fluffy, 4 minutes.
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7
Beat in the eggs, one at a time, scraping down the bowl.
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8
Beat in the dry ingredients in 3 batches, alternating with the milk mixture and scraping down the bowl.
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9
Mix 2 cups of the batter with the melted and cooled bittersweet chocolate.
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10
Scrape the remaining batter into the pans.
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11
Dollop the chocolate batter over the batter in the pans and swirl with a knife.
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12
Bake in the lower third of the oven for about 35 minutes, until springy.
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13
Let cool for 15 minutes, then run a knife around the edges and invert onto a rack.
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14
Peel off the paper, turn the cakes upright and cool completely.
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15
Melt the bittersweet chocolate in 1/4 cup of the heavy cream and let cool.
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16
In a standing mixer fitted with the whisk, beat the butter and egg yolk at medium-high speed until creamy.
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17
Beat in the sugar at low speed.
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18
Beat in the remainder of the cream, vanilla, and salt, then beat at medium-high speed until fluffy, 3 minutes longer.
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19
Beat in the cooled chocolate cream.
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20
Spread 1 cup of frosting over a cake layer and top with the second layer; frost the top and sides.
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21
Let stand 1 hour; serve.
