Make the Bailey's Chocolate Mousse:
Sprinkle gelatin in a small bowl. Add cold water and stir briefly. Let stand until the gelatin has soaked up the water,
about 1 minute or so.
Add boiling water to the bowl and stir until the gelatin is completely dissolved. Set aside to cool completely.
In a bowl of an electric stand mixer fitted with a whisk attachment (a hand mixer works wonderfully here too) combine the sugar and the cocoa powder. Add the heavy cream and mix over medium/high speed until the mixture thickens. In a slow stream pour the Baileys, followed by the vanilla extract, and finally the gelatin mixture.
Increase the speed to high and continue whisking until thickened, about 1 minute or so. Remove bowl from mixer and test thickness by dipping the whisk into the mousse and lifting. If the mousse does not immediately dissolve onto itself it's ready. You want the mousse thick enough to form shapes that will not dissolve easily.
Spoon the mousse into a container with a tight-fitting lid and place in the freezer to chill for at least one hour.
Bake the cake:
Preheat your oven to 350 degrees Fahrenheit (180 degrees C) and place the oven rack in the center of the oven. Butter or spray with a nonstick vegetable spray, three - 8 x 2 inch deep round baking pans. Line the bottoms of the pans with
parchment paper and set aside. Add height to your spring-form pan by lining it with a ring of parchment paper about 2-3 inches deep and set aside.
In a small heatproof bowl, place the finely chopped unsweetened chocolate and cocoa powder. Add the hot coffee and stir until the mixture is melted and smooth. Set aside to cool to room temperature. To a medium bowl, add the flour, baking powder, baking soda, and salt. Stir quickly with a fork to combine.
In the bowl of your electric mixer, fitted with a paddle attachment (a hand mixer works here as well) over medium/high speed, beat the butter and sugars until the mixture is light and fluffy, about 3-4 minutes. Add the eggs one at a time beating well after each addition. Stop the mixer and scrape the sides of the bowl. Turn the mixer to low/medium speed. Add the vanilla extract and melted chocolate mixture and beat to combine. The mixture should look sort of billowy, like a mousse.
Reduce the speed to low. Add the flour mixture alternating with the milk, beginning and ending with the flour mixture. Beat until the ingredients are incorporated, but be sure not to over mix or the cake will be tough.
Divide the batter evenly between the three prepared pans. Bang the pans on your counter to smooth out the batter and remove any air bubbles. Place in the oven and bake for about 25-30 minutes or until a toothpick inserted in the center comes out clean and the cake starts to separate from the sides.
Remove pans from oven and place on a wire rack to cool for about 8-10 minutes. Carefully invert the cakes (one at a time) over the wire rack and lift the pan. Remove the parchment paper and then, to prevent cracks, reinvert the cake onto a wire rack so that tops are right side up. Allow the cakes to cool completely. While the cakes bake, make the chocolate buttercream frosting.
Make the chocolate buttercream frosting:
In a bowl of an electric mixer fitted with a whisk attachment, whip butter, cocoa powder, and salt over medium/high speed until combined. Turn the mixer off and add the confectioners' sugar all at once. Turn the mixer on to the lowest speed and whip until the confectioners' sugar is completely combined with the butter/cocoa powder mixture. Do not start the mixer at high speed or you will have confectioners' sugar all in your faces.
Once the mixture is combined, increase the speed to medium/high and continue whipping until combined, about 3-5 minutes. The mixture will looks a bit sandy and that’s fine. Stop and scrape the mixer to incorporate.
Turn the mixer to medium/high speed and add the heavy cream in 2 parts, allowing the heavy cream to thicken each time before adding in more cream. Add the vanilla extract and continue whipping the frosting until very light and fluffy, about 3 more minutes.
Turn off mixer and remove bowl. Cover with plastic wrap and place in the refrigerator until ready to use.
Grab your collared springform pan. Begin by placing a layer of chocolate cake on the bottom. Spread half (or less) of the Baileys chocolate mousse, smoothing flat with an offset spatula. If the mousse drapes over the cake, it's cool. Place a layer of chocolate cake over the mousse. Spread remaining Baileys mousse and smooth flat with an offset spatula. Top with final layer of chocolate cake. Cover the cake and place in the freezer to set completely, about 3 hours.
Remove cake from freezer and chocolate buttercream from the fridge. Frost the cake with the chocolate buttercream, smoothing with an offset spatula as best you can. Place frosted cake in the freezer while you make the ganache.
Make the ganache:
Pour the chocolate in a medium bowl and set aside. In a small saucepan, bring the heavy to a simmer. Pour the scalding heavy cream over the chocolate and stir until combined. Allow to cool for a few minutes. Remove cake from freezer and pour ganache evenly over frosted cake.
Bailey's Chocolate mousse adapted from: Lemon Tree Dwelling