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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 them with parchment paper; butter and flour the paper, shaking off the excess.
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3
In a medium bowl, mix the flour, baking powder and salt.
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4
In a cup, mix the milk with the vanilla extract.
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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 granulated sugar and beat until fluffy, about 4 minutes.
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7
Beat in the eggs, 1 at a time, scraping down the bowl between additions.
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8
Beat in the dry ingredients in 3 batches, alternating with the milk mixture.
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9
Transfer 2 cups of the batter to a small bowl.
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10
Scrape the remaining cake batter into the prepared cake pans.
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11
Stir the melted chocolate into the reserved batter; dollop the batter over the plain batter and swirl it in with a butter knife.
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12
Bake the cakes in the lower third of the oven for 35 to 40 minutes, until a toothpick inserted in the center comes out with a few moist crumbs attached.
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13
Let the cakes cool in the pans for 15 minutes.
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14
Run a knife around the edges and invert onto a rack.
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15
Peel off the paper, turn the cakes right side up and let cool completely.
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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 confectioners' sugar at low speed.
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18
Beat in 6 tablespoons of the cream, the vanilla and the salt, then beat at medium-high speed until fluffy, about 3 minutes.
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19
In a small saucepan, bring the remaining 1/4 cup of heavy cream to a boil.
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20
Remove from the heat, add the chopped chocolate and let stand until the chocolate is melted, about 3 minutes.
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21
Whisk until smooth, then transfer to a small bowl and let cool.
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22
Beat the chocolate into the frosting.
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23
Spread 1 cup of the frosting over a cake layer and top with the second cake layer; frost the top and side.
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24
Let stand for 1 hour before serving.
