Bring stout and butter to simmer on medium heat. Remove from heat once butter is melted. Whisk in cocoa powder off heat and set aside to cool slightly.
In a medium bowl, whisk together flour, sugar, baking soda and salt.
Using mixer, beat eggs and sour cream on low until smooth and fully incorporated. Slowly add chocolate mixture (pour down side of bowl) while continuing to beat on low. Scrape down bowl to make sure everything is uniformly combined.
Add flour mixture to egg and chocolate mixture and mix on low speed until well combined. Scrape down bowl and fold batter until combined.
Pour batter into prepared pan and bake in preheated oven until tester comes out clean, approximately 45 minutes.
Cool in pan on rack for 10 minutes, then turn out onto serving plate. Allow to cool completely.