Preheat oven to 325 degrees F (165 degrees C). Grease three 9-inch baking pans with cooking spray. Line bottoms with parchment paper; grease paper.
Place 6 ounces chocolate in a small bowl. Pour coffee over the chocolate and whisk until smooth. Let stand until slightly cooled, about 5 minutes.
Whisk flour, 1 cup cocoa powder, baking soda, salt, and baking powder together in a bowl.
Beat eggs in a stand mixer on high speed until lemon-colored. Beat in white sugar, coffee mixture, oil, and vanilla extract gradually until batter is well blended. Add flour mixture alternately with buttermilk, beating well after each addition.
Divide batter evenly among the prepared baking pans.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Cool in the pans for 10 minutes. Invert onto cooling racks and let cool completely, about 30 minutes.
Pour 2 1/2 cups heavy cream into a small saucepan; heat until it just boils.
Place 20 ounces chocolate in a large bowl; pour hot cream on top and whisk until smooth. Let stand at room temperature, stirring occasionally, until slightly thickened, about 45 minutes. Refrigerate until ganache is set and spreadable, 4 to 5 hours.
Combine 1/4 cup ganache, confectioners' sugar, butter, and 1/2 cup cocoa powder in a stand mixer fitted with a paddle attachment; beat on low speed until smooth. Increase speed to high and beat until buttercream is smooth and silky.
Place one cake layer on a serving plate; spread 1/3 cup ganache on top. Repeat twice more. Spread remaining ganache around the sides of the cake. Refrigerate until set, about 30 minutes.
Spread buttercream over the top and sides of the cake. Refrigerate until set, about 10 minutes.
Pour 1 cup heavy cream into a small saucepan; heat until it just boils.
Place 1 cup chocolate chocolate in a small bowl; pour hot cream on top and whisk until smooth. Drizzle glaze over cake. Refrigerate until glaze firms up, at least 30 minutes.