Preheat oven to 350°F and spray three 6-inch cake pans (or two 8-inch cake pans) with nonstick baking spray.
In a large bowl, whisk together sugar, flour, cocoa powder, baking powder, and salt. Add eggs, milk, stout beer, oil, and vanilla. Mix on medium speed with an electric mixer until all ingredients are incorporated, about 1 to 2 minutes.
Divide batter evenly among the prepared cake pans and bake for 18 to 20 minutes, or until center is set and a wooden pick inserted into the center of each cake comes out clean. Let cakes cool in pans for 10 to 15 minutes, then transfer to a wire rack to cool completely. If cake layers are domed or aren’t smooth on the top, use a serrated knife or cake saw to create a flat top for each layer.
Chocolate Stout Ganache:
In a microwave safe bowl, combine chocolate chips, cream, and stout beer. Microwave in 30 second intervals, stirring well between each, until melted and smooth. Chill ganache in the refrigerator for at least 30 minutes. (Or you can make the ganache a day in advance and store covered in the refrigerator.)
Chocolate Stout Frosting:
In the bowl of a stand mixer with the paddle attachment, beat butter on medium speed until smooth and creamy. Add cocoa powder, half of the powdered sugar, and 3 tablespoons beer. Mix on low speed until mostly combined, and then medium-high speed until completely combined. Scrape the sides of the bowl with a rubber spatula and add remaining powdered sugar. Mix on low speed until mostly combined, and then medium-high speed until completely combined. Add another tablespoon if frosting is too thick.
To assemble the cake, place one layer on a cake stand or cake plate. Spoon some frosting into a piping bag and pipe a border around the top of the cake layer. Spoon ganache into the center and spread with a spoon until the whole border is filled. Repeat this process with the second cake layer. (You may have a couple of tablespoons of ganache leftover that you can use to drizzle over the top of your finished cake.)
Place the third cake layer on top of that and spread frosting around the outside and top of the cake, covered and decorating as you wish! I piped some frosting over the top of the cake, drizzled with ganache, and sprinkled with gold sugar sprinkles.
Serve and enjoy! (Or store cake in refrigerator until ready to serve.)