Add the Guinness and butter to a medium saucepan, bring to a simmer over medium heat. Add cocoa and whisk until smooth. Cool. Whisk flour, sugar, baking soda, and salt in a large bowl. Beat the eggs and sour cream in the bowl of an electric mixer. Add the Guinness mixture and beat until combined. Add the flour mixture and mix until just combined.
Set EGG for indirect cooking (with convEGGtor) to 350°F/177°C. Divide the batter evenly among two muffin tins lined with paper liners and cook in EGG for 17 to 20 minutes. Cool completely on a wire rack.