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1
Preheat the oven to 350.
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2
Butter a 12-by-16-inch rimmed baking sheet and line the bottom with parchment paper.
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3
In a large bowl, sift the flour with the cocoa, baking powder, baking soda and salt.
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4
Whisk in the sugar.
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5
In a medium bowl, whisk the buttermilk with the eggs, coffee, butter and vanilla.
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6
Whisk the wet ingredients into the dry ingredients until blended.
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7
Scrape the batter onto the prepared baking sheet and smooth the top.
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8
Bake for 22 minutes, or until a tester inserted in the center comes out clean.
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9
Transfer the baking sheet to a rack and let cool for 30 minutes, then freeze until firm, about 1 hour.
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10
Bring a medium saucepan filled with 1 inch of water to a simmer.
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11
Combine the bittersweet and unsweetened chocolate in a heatproof bowl and set the bowl overnot inthe simmering water to melt the chocolate; let cool slightly.
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12
In a large bowl, using an electric mixer, beat the butter and salt at medium speed until smooth.
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13
Beat in the rum, vanilla and melted chocolate until fully incorporated.
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14
Beat in the confectioners' sugar one-third at a time, scraping down the bowl between additions.
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15
Using a rubber spatula, fold in the sour cream until no white streaks remain.
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16
Refrigerate the frosting until ready to use.
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17
Spread the almonds on a rimmed baking sheet and bake for 8 minutes, or until lightly toasted.
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18
Transfer to a bowl and let cool.
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19
Slice the frozen cake in half crosswise and transfer one half to a serving plate.
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20
Spread one-third of the frosting on the cake and top with the other half of the cake.
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21
Refrigerate for 30 minutes, until the frosting is set.
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22
Spread the remaining frosting evenly over the top and sides of the cake and refrigerate for 1 hour longer, or until the frosting is set.
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23
One hour before serving, remove the cake from the refrigerator.
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24
Toss the chopped toffee bars with the toasted almonds and gently press them into the top and sides of the cake.
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25
Slice and serve.