This three-tiered beauty is so much easier to make than it looks. The frosting is best made the day before you need to use it, as it sets to the right spreading consistency when left to cool overnight at room temperature. A splash of bourbon enhances the caramel flavors in the creamy frosting, but it can be left out without causing any problems.
Prepare the Chocolate-Caramel Ganache: Stir together chocolate chips, corn syrup, and, if desired, bourbon in a large bowl. Set aside.
Stir together sugar, cold water, and cream of tartar in a large, heavy saucepan. Cook over medium, swirling pan occasionally, until sugar dissolves and starts to change color. Increase heat to high, and cook, swirling pan occasionally, until mixture is a rich, golden caramel color, 4 to 5 minutes.
Remove pan from heat. Using a long- handled wooden spoon, carefully stir in heavy cream. (Be careful: Hot caramel can spit and bubble as the cream hits it.) Reduce heat to low, and return pan to heat. Cook, stirring constantly, until caramel thickens slightly and any lumps of hardened caramel have melted, about 4 minutes. Pour warm caramel sauce over chocolate chip mixture, covering chips completely. Let stand 1 minute. Stir until mixture is completely smooth. Stir in salt. Add butter, 1 tablespoon at a time, stirring until melted and combined after each addition. Let ganache cool until lukewarm. Cover loosely with plastic wrap, and let stand at room temperature 8 hours or overnight.
Prepare the Cake: Preheat oven to 350°F. Lightly coat 3 (9-inch-round x 2-inch-deep) cake pans with cooking spray, and line bottoms with parchment paper. Sift together flour, cocoa, baking powder, baking soda, and salt in a large bowl. Set aside.
Beat sugar, sour cream, oil, eggs, and vanilla in a large bowl with an electric mixer on medium speed until thick and smooth, about 2 minutes. Add flour mixture, one-third at a time, to butter mixture. Beat on low speed just until combined after each addition. (Do not overbeat or the cake will be tough.) Add 3/4 cup of the hot coffee, and beat just until smooth. Add remaining 3/4 cup coffee, and beat just until smooth. Divide batter evenly among prepared pans.
Bake in the middle of preheated oven until a long wooden pick or cake tester inserted in center of cake comes out clean, 20 to 25 minutes. Cool in pans on a wire rack 10 minutes. Invert layers onto wire rack, and peel off parchment paper. Cool completely, about 45 minutes.
Place 1 cake layer on a serving plate; spread evenly with about 1 heaping cup of Chocolate-Caramel Ganache. Top with second cake layer, and spread evenly with 1 heaping cup of ganache. Top with remaining cake layer, and frost top and sides of cake with remaining ganache. Let stand at room temperature until ready to serve, or cover and refrigerate overnight.