Preheat the oven to 350°F degrees. Set Flexipan Mold or Molds of choice on Perforated Baking Sheet and set aside.
Pour stout into a saucepan and add butter; bring to a simmer over medium-low heat.
Remove the pan from heat, using Mini Whisk, whisk in cocoa powder until smooth. Set aside to cool slightly while you prepare the rest of the cake batter.
Beat eggs and sour cream with your mixer until well combined. Stir in stout mixture until smooth.
Mix flour, sugar, baking soda, and salt together in a separate Stainless Mixing Bowl. Pour stout mixture into flour mixture and gently combine with Blender Spatula until smooth. Pour batter into Flexipan Molds.
Bake until a toothpick inserted into the center of a cake comes out clean, about 30 minutes (less time if using multiple molds and more time if using one large mold). Let cake cool completely before unmolding.