Center a rack in the oven and preheat to 350 (F). Prepare a muffin pan by lining with 12 cupcake liners. In a medium bowl, whisk together 3/4 cup all-purpose flour, 3/4 cup granulated sugar, 3/4 teaspoon baking soda, and 1/2 teaspoon kosher salt. Set aside.
In a medium saucepot, melt 1/2 cup unsalted butter. Whisk in 1/3 cup natural unsweetened cocoa powder and 1/2 cup stout beer. Let the mixture cool slightly.
In a large bowl, whisk together 1 large egg and 1/3 cup sour cream. While whisking the mixture, add the butter, cocoa powder, and beer mixture. Continue whisking until well combined.
Sprinkle the dry ingredients (from the 2nd step) over the wet ingredients (from the 4th step) and continue whisking until smooth.
Spoon the batter into the prepared muffin pan — do NOT fill the cups more than two-thirds full, otherwise, they'll overfill and sink in the center! If necessary, reserve some batter for a second batch.
Bake in the preheated oven for 20 to 25 minutes, until a toothpick inserted into the center of a cupcake comes out clean. Transfer to a wire rack to cool for 10 minutes, before turning out to cool completely before frosting.