Preheat the oven to 350 degrees. Spray 2 9-inch round cake pans with cooking spray. Line the bottoms of the pans with parchment, then spray the parchment with cooking spray. Lightly dust with flour and set pans aside.

Combine the stout and butter in a medium saucepan set over medium heat and bring to a simmer. Remove pan from the heat and whisk in the cocoa powder until mixture is smooth. Set aside to cool for 5 minutes.

Whisk the flour, sugar, baking soda, and salt together in a large mixing bowl. In a separate large bowl, whisk the sour cream and eggs together.

Stir the stout mixture into the sour cream mixture and whisk until combined. Fold the flour mixture into the stout mixture with a rubber spatula until the batter is smooth and free of lumps.

Divide batter evenly between the prepared pans and bake 35 to 40 minutes, until a toothpick inserted into the center of the cakes comes out clean. Cool completely on a wire rack before frosting.

Place the butter in the bowl of a stand mixer fitted with the paddle attachment and beat until very creamy. Add 3 cups of the sugar and 1/4 cup of the heavy cream and mix on low until combined. Turn the mixer to medium-high speed and beat until very creamy, about 1 minute more. Scrape down the sides and bottom of the bowl.

With mixer on low, beat in the remaining sugar 1 cup at a time until the frosting is thick but still spreadable; if you add too much sugar just add more heavy cream. Once frosting is your desired consistency, add the vanilla bean seeds, vanilla extract, and salt. Turn mixer to high speed and beat until frosting is very light and fluffy, about 3 or 4 minutes.

Place one of the cooled cake layers on a cake stand or cardboard cake circle and spread a thick layer of frosting over the top. Cover with the remaining cake layer and frost with a very thin layer of frosting. Refrigerate the cake to set the frosting (this will prevent cake crumbs from accumulating in your final coat of frosting). Frost with remaining frosting and decorate as desired.