Tiramisu Layer Cake

Preheat the oven to 350 degrees F (175 degrees C). Grease and flour three 9-inch pans.

Make cake: Beat cake mix, water, egg whites, and oil in a large bowl with an electric mixer on medium speed until well combined, about 2 minutes. Divide 2/3 of the batter between 2 prepared pans. Stir instant coffee into remaining batter; pour into the remaining prepared pan.

Bake in the preheated oven until a toothpick inserted into the center of cakes comes out clean, 23 to 28 minutes. Cool cakes in the pans for 10 minutes, then turn out onto a wire rack to cool completely.

Make syrup: Combine brewed coffee and coffee liqueur in a measuring cup; set aside.

Make filling: Beat mascarpone, confectioners' sugar, and coffee liqueur in a medium bowl with an electric mixer on low speed until smooth. Cover with plastic wrap and place in the refrigerator to keep cool.

Make frosting: Beat cream, confectioners' sugar, and coffee liqueur in a medium bowl with an electric mixer on medium-high speed until stiff. Fold 1/2 cup frosting into chilled filling until combined. Store frosting and filling, covered, in the refrigerator to keep cool.

Assemble cake: Place 1 plain cake layer on a serving plate. Using a thin skewer, poke holes in cake, about 1 inch apart. Pour 1/3 of the coffee syrup over cake, then cover with 1/2 of the filling. Top with coffee-flavored cake layer; poke holes in cake. Pour another 1/3 of the coffee syrup over cake and cover with remaining filling. Top with remaining cake layer; poke holes in cake. Pour remaining coffee syrup on top. Spread sides and top of cake with frosting.

Garnish cake: Place cocoa powder in a sieve and lightly dust top of cake. Run a vegetable peeler down the edge of chocolate square to form curls. Scatter chocolate curls over cake.

Chill cake in the refrigerator for at least 30 minutes before serving.