Note: I used three 9-inch cake pans, but mine were only 1 1/2 inches deep. If you have cake pans that are 2 inches+ deep, you can use two 9-inch or 3 8-inch pans. Grease your pans or spray with non-stick baking spray.
Preheat the oven to 350 degrees F.
Combine the beer and butter in a saucepan over medium-low heat. Once the butter melts, whisk in the cocoa powder until smooth. Let the mixture cool to room temperature.
In a large bowl, whisk together the flour, sugar, baking powder and salt.
In the bowl of an electric mixer, beat together the eggs and yogurt. Add in the chocolate stout mixture, beating until combine. Gradually add in the flour mixture until just combined, beating for only about a mixture.
Divide the batter between the baking pans. I like to use a 1-cup measure to get equal scoops as well as my kitchen scale to make sure it's the same weight. This will guarantee those even leveled layers.
Bake the cakes for 30 to 35 minutes (for 8 inch) or a little longer for 9 inch - until a tester in the center comes out clean. Let the cakes cool in their pans for 10 to 15 minutes, then gently invert them on parchment paper. Let cool completely before frosting.
To assemble, place the bottom layer of cake on the cake stand or plate that you'd like to use. Add a bit of the frosting (about 3 to 4 tablespoons if you have 3 layers, or more if you only have 2) and spread it to the edges. Repeat with another layer of cake. If you have a third layer, add it on top and spread the rest of the frosting on the top and down the sides. I like to smooth it out with a large spatula! Sprinkle the cake with the flaked sea salt and serve. You can also keep it chilled in the fridge (I recommend that!) and it's unreal.