Preheat the oven to 350 degrees F (175 degrees C). Line two 9-inch cake pans with parchment paper and spray them both well with cooking spray.
Set up a stand mixer with the paddle attachment and combine the All-Purpose Flour (2 cup), Granulated Sugar (1 cup), Dark Brown Sugar (1 cup), Unsweetened Cocoa Powder (1 cup), Baking Powder (2 teaspoon), Baking Soda (1 1/2 teaspoon), Ground Cinnamon (1/2 teaspoon), and Fine Salt (1 pinch) in its bowl. Mix them together with the paddle just to aerate them.
Slowly pour in the Milk (1 cup) and Vegetable Oil (1/2 cup), followed by the Egg (2) and Chocolate Liqueur (2 teaspoon). Very slowly and carefully pour in the Water (1 cup) with the mixer on low.
Evenly divide the batter among the two prepared pans, then bake them for 30-35 minutes. A toothpick inserted in the center should come out completely clean.
Let them cool in the pans for 15 minutes or so, then turn them out of the pans to cool completely. Clean out the stand mixer bowl and paddle since you will need them for the buttercream.
Fill a small saucepan halfway with water and bring it to a low boil. Combine the whites of the Egg (4), Granulated Sugar (1 1/2 cup), and Salt (1 pinch) in a heatproof bowl and set it over the simmering water while you whisk it. Keep whisking while it gently cooks for a couple of minutes.
Transfer it into the stand mixer bowl and use the whisk attachment to whip it up to be stiff peaks for a few minutes. It should be white and shiny. Switch to the paddle attachment and slowly add in the Butter (2 1/4 cup) a stick at a time, finishing with the half stick. Finally, mix in the Vanilla Extract (1 teaspoon) to finish it up! It will be a luscious buttercream.
Place the first layer on a cake plate and spread a thick layer of the frosting on it. Place the second layer on top and generously frost the top and sides of the cake. Decorate it to your heart's desire with edible beads, sprinkles, you name it! Enjoy!