Directions

1. Preheat oven to 350 degrees. Butter a 12-by-17-inch rimmed baking sheet, and line with parchment paper. Butter lining. Dust bottom and sides with flour; tap out excess.
2. Whisk together flour, baking soda, and salt in a medium bowl; set aside. Whisk together chocolate, coffee, and boiling water in a separate bowl; set aside.
3. Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; beat on medium-high speed until pale and fluffy. Add sour cream; beat until smooth. Beat in eggs, one at a time, scraping down sides of bowl. Reduce speed to low; beat in flour mixture.
4. Whisk chocolate mixture again; add to batter. Beat until smooth.
5. Pour batter into prepared sheet. Bake until a cake tester inserted into center comes out clean, 20 to 25 minutes. Let cool completely in sheet on a wire rack.
6. Invert cake onto a rimless baking sheet; peel off parchment. Halve widthwise. Spread half of buttercream on top of one layer. Top frosted layer with second cake layer; coat cake with remaining frosting. Refrigerate 1 hour. Cut into 12 squares; garnish each serving with a caramelized cobweb.