How to Make It
Preheat the oven to 350°. Butter a 12-by-16-inch rimmed baking sheet and line the bottom with parchment paper. In a large bowl, sift the flour with the cocoa, baking powder, baking soda and salt. Whisk in the sugar.
In a medium bowl, whisk the buttermilk with the eggs, coffee, butter and vanilla. Whisk the wet ingredients into the dry ingredients until blended. Scrape the batter onto the prepared baking sheet and smooth the top. Bake for 22 minutes, or until a tester inserted in the center comes out clean. Transfer the baking sheet to a rack and let cool for 30 minutes, then freeze until firm, about 1 hour.
Bring a medium saucepan filled with 1 inch of water to a simmer. Combine the bittersweet and unsweetened chocolate in a heatproof bowl and set the bowl over—not in—the simmering water to melt the chocolate; let cool slightly.
In a large bowl, using an electric mixer, beat the butter and salt at medium speed until smooth. Beat in the rum, vanilla and melted chocolate until fully incorporated. Beat in the confectioners' sugar one-third at a time, scraping down the bowl between additions. Using a rubber spatula, fold in the sour cream until no white streaks remain. Refrigerate the frosting until ready to use.
Spread the almonds on a rimmed baking sheet and bake for 8 minutes, or until lightly toasted. Transfer to a bowl and let cool.
Slice the frozen cake in half crosswise and transfer one half to a serving plate. Spread one-third of the frosting on the cake and top with the other half of the cake. Refrigerate for 30 minutes, until the frosting is set. 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.
One hour before serving, remove the cake from the refrigerator. Toss the chopped toffee bars with the toasted almonds and gently press them into the top and sides of the cake. Slice and serve.