Special equipment: two 8-inch round cake pans; a candy thermometer; a pastry bag
For the cake: Preheat the oven to 350 degrees F and grease two 8-inch round cake pans.
Whip the granulated sugar and eggs with a stand mixer fitted with the whisk attachment. Meanwhile, put the coffee, butter, vinegar and 1/2 cup of the chocolate chips into a microwave-safe bowl. Microwave until the chocolate and butter are melted, about 3 minutes; then whisk together until well-blended. In a separate mixing bowl, add the flour, cocoa powder, baking soda and salt; whisk together. Add the oil and sour cream to the bowl with the chocolate mixture and mix well.
Add a third of the flour mixture to the egg mixture and mix well to incorporate. Then mix in half of the chocolate mixture. Continue to alternate with the flour and chocolate mixtures, scraping down the sides of the bowl occasionally and finishing with the flour. Divide the batter between the prepared cake pans. Fill each pan slightly below the top edge (you may not use all the batter). Sprinkle the remaining 1/2 cup chocolate chips over the top of each. Bake until the centers bounce back when touched, 30 to 35 minutes.
For the toffee sauce: Melt together the butter and brown sugar in a heavy saucepot and cook until simmering. Remove from the heat, then add the heavy cream and vanilla, and stir. Return to the heat and allow the sauce to boil for another 3 minutes. Remove from the heat and pour into a metal mixing bowl. Cool completely, then refrigerate.
For the chocolate ganache: Place the chocolate chips, heavy cream and butter in a microwave-safe bowl and microwave for 3 minutes. Stir until all the chocolate is melted and the ingredients are incorporated. Cool in the fridge until the consistency is like a paste.
For the buttercream: Add the egg yolks to a mixing bowl and whip with an electric mixer until light.
In the meantime, mix the sugar and 1/2 cup water in a heavy saucepan and bring to a boil. Wet a pastry brush with water and wash down the sides of the pan to avoid crystallization. Continue to cook until the sugar syrup registers 235 degrees F (softball stage) on a candy thermometer. Remove from the heat and, with your mixer running constantly at medium speed, immediately but slowly pour the hot sugar into the egg yolks in a steady stream.
Continue to beat with the mixer until the mixture cools down completely. Add the butter, vanilla extract and vanilla bean seeds. Continue to beat until all of the butter is incorporated and the icing is smooth, glossy and spreadable (see Cook's Note). Add some buttercream to a pastry bag.
To assemble the cake: Place 1 cake layer on a cake plate. With the pastry bag, pipe a border around the top edge of the cake layer for a "frosting dam" to hold in the toffee sauce. Poke holes into the top of the cake layer and pour at least 1/4 cup toffee sauce (or more to taste) over the top so that it penetrates the cake. Then spread a thin layer (about 1/8-inch thick) of ganache over the top. Put in the freezer for 5 minutes to make the ganache solidify. Then spread a layer of buttercream over the top of the chilled ganache. Generously sprinkle some English toffee bits over the buttercream and drizzle with more toffee sauce and ganache. Place the second cake layer on top and ice with the buttercream. Drizzle the top with more ganache and toffee sauce, then sprinkle toffee bits over the top. Serve immediately or refrigerate, covered. Take out of the fridge 1 hour prior to serving, to bring to room temperature.
If your icing is too watery, then the yolks did not cool down properly. Remedy that by allowing it to cool, then adding another stick or two of butter and beating thoroughly.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.