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1
Preheat the oven to 350.
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2
In a small bowl, whisk together 2 tablespoons each of the flour and cocoa powder.
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3
Butter two 9-inch round cake pans and line the bottoms with parchment paper.
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4
Butter the paper and dust the pans with the cocoa-flour mixture, tapping out the excess.
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5
In a medium bowl, whisk the 2 cups of flour and 1/4 cup of cocoa powder with 1/2 cup of the granulated sugar, and the baking powder, baking soda and salt.
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6
In the bowl of a standing electric mixer fitted with the whisk, beat the eggs with the brown sugar and the remaining 1/2 cup plus 1 tablespoon of granulated sugar at medium-high speed until thickened, about 5 minutes.
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7
At medium speed, gradually add 1/2 cup of the oil and beat for 3 minutes.
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8
Beat in the melted chocolate, then add the remaining oil in a thin stream, until thoroughly blended, scraping the side and bottom of the bowl.
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9
At low speed, beat in the buttermilk and vanilla.
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10
Remove the bowl from the mixer and gently mix in the dry ingredients by hand.
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11
Pour the cake batter into the prepared pans and bake in the center of the oven for about 25 minutes, until a toothpick inserted in the centers of the cakes comes out with a few moist crumbs attached.
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12
Cool the cakes in the pans for 20 minutes, then invert them onto a rack to cool completely.
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13
Peel off the parchment paper.
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14
Set one cake layer on a platter.
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15
Spread the mousse filling on top and cover with the second cake layer.
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16
Frost the cake all over with a thin layer of buttercream and refrigerate until set, about 5 minutes.
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17
Frost the cake with the remaining buttercream.
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18
Refrigerate the cake until the frosting is firm, at least 15 minutes, before serving.
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19
Set one cake layer on a platter.
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20
Spread the mousse filling on top and cover with the second cake layer.
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21
Frost the cake all over with a thin layer of buttercream and refrigerate until set, about 5 minutes.
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22
Frost the cake with the remaining buttercream.
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23
Refrigerate the cake until the frosting is firm, at least 15 minutes, before serving.