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1
Preheat the oven to 350 degrees F.
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2
Butter 1 (4 by 3-inch) cake pan, 1 (6 by 3-inch) cake pan and 1 (8 by 3-inch) cake pan.
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3
Line the bottoms with parchment paper, and butter and flour the pans.
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4
Sift the flour, cocoa, baking soda, and salt together in a medium bowl.
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5
In the bowl of an electric mixer fitted with a paddle attachment, cream the butter and sugars on high speed until light, approximately 5 minutes.
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6
Add the eggs and vanilla and mix well.
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7
Combine the buttermilk, sour cream, and coffee.
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8
On low speed, add the flour mixture and the buttermilk mixture alternately in thirds, beginning with the buttermilk mixture and ending with the flour mixture.
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9
Mix the batter only until blended.
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10
Divide the batter between the 3 pans and smooth the tops with a spatula.
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11
Bake on the middle rack of the oven as follows: for the 4-inch pan 45 to 50 minutes, or until a toothpick comes out clean; for the 6-inch pan 70 to 75 minutes, or until a toothpick comes out clean; and for the 8-inch pan 85 to 90 minutes, or until a toothpick comes out clean.
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12
Cool the cakes in the pans for 10 minutes each on a cooling rack, carefully remove from the pans and allow to finish cooling.
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13
Place the 8-inch cake on a serving plate, flat side up.
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14
Frost the top of that layer with buttercream.
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15
Place the 6-inch cake in the center of the top of the 8-inch cake, flat-side up, and frost the top and sides.
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16
Place the 4-inch cake in the center of the top of the 6-inch cake, flat side down.
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17
Frost the top and sides of that layer with buttercream.
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18
Spread any remaining buttercream over the cake evenly.
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19
Chop the chocolates and place them in a heat-proof bowl set over a pan simmering water.
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20
Stir until melted and set aside until cooled to room temperature.
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21
Mix the egg whites, sugar, cream of tartar, and salt in the bowl of an electric mixer fitted with a whisk attachment.
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22
Place the bowl of egg whites over the pan of simmering water and heat the egg whites until they are warm to the touch, about 5 minutes.
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23
Return the bowl to the electric mixer and whisk on high speed for 5 minutes, or until the meringue is cool and holds a stiff peak.
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24
Add the butter, 1 tablespoon at a time, while beating on medium speed.
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25
Scrape down the bowl, add the melted chocolate, vanilla, espresso, and rum, if using, and mix for 1 minute or until the chocolate is completely blended in.
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26
If the buttercream seems very soft, allow it to cool, and beat it again.