Chocolate Fudge Layer Cake with Caramel Buttercream

Prepare the Fudge Cake: Coat 2 (9-inch) square cake pans with cooking spray; lightly dust with flour. Preheat oven to 350°F. Place chocolate chips in a microwavable bowl; microwave on MEDIUM (50% power) until melted, about 2 minutes, stopping to stir every 30 seconds. Stir melted chocolate until completely smooth.

Beat butter and brown sugar in bowl of a heavy-duty stand mixer on medium speed until well combined, about 5 minutes. Add eggs, 1 at a time, beating until just combined after each addition. Add melted chocolate, beating until just combined.

Sift together flour, baking soda, salt, and cinnamon in a bowl. Gradually add to melted-chocolate mixture alternately with sour cream, beginning and ending with flour mixture, beating on low speed until just blended after each addition. Gradually add coffee, beating on low speed until just blended. Stir in bourbon. Pour batter evenly into prepared cake pans.

Bake cake layers in preheated oven until a wooden pick inserted in centers comes out clean, 22 to 25 minutes. Cool in pans on wire racks 10 minutes. Remove cake layers from pans; transfer to wire racks to cool completely, about 1 hour.

Prepare the Caramel Buttercream: Place granulated sugar and water in a small saucepan; cook, stirring often, over medium-high, until sugar dissolves, about 1 minute. Bring to a boil over medium-high. Cook, without stirring but swirling pan occasionally, until mixture is deep amber in color, about 10 minutes. Remove from heat; quickly add cream in a thin, steady stream, stirring constantly with a wooden spoon. Stir in chilled butter until mixture is smooth. Transfer caramel to a small bowl to cool completely, about 1 hour.

Beat softened butter and cream cheese in bowl of a heavy-duty stand mixer on medium speed until creamy, 3 minutes. Stir in vanilla and salt. Gradually add 2 cups of the powdered sugar, beating on low speed until smooth, 2 minutes. Add cooled caramel; beat on medium speed until combined, 2 minutes. Gradually add remaining 2 cups powdered sugar, beating on low speed until combined.

Prepare the Chocolate Ganache: Place semisweet and bittersweet baking chocolates and cream in a microwavable bowl; microwave on MEDIUM (50% power) for 1 minute. Remove and stir. Microwave until melted, 3 to 3 1⁄2 minutes, stopping to stir every 30 seconds.

Assemble the cake: Place 1 cooled cake layer on a serving platter; pour half of ganache over top, allowing some to drip down sides. Freeze until just set, about 5 minutes. Dollop with half of buttercream; spread gently to edges of cake layer. Top with remaining cake layer; repeat process with remaining ganache and buttercream. Garnish with shaved chocolate.