In a large bowl, using a handheld mixer on medium-high speed, beat cream, confectioners' sugar, and sour cream until soft peaks form. Refrigerate until ready to use.

In a medium saucepan over medium heat, bring Guinness to a simmer and cook, stirring occasionally, until reduced by about three-quarters (you should have a little more than ⅓ cup), about 15 minutes. Let cool.

Meanwhile, arrange a rack in center of oven; preheat to 425°. Grease a standard 12-cup muffin tin with butter, then dust with 2 Tbsp. granulated sugar, tapping out excess.

In a medium microwave-safe bowl, microwave chocolate and 6 Tbsp. butter in 15-second increments, stirring between each, until melted and smooth, about 45 seconds total.

In a large bowl, using handheld mixer on medium-high speed, whisk eggs, egg yolks, vanilla, and remaining ¾ cup granulated sugar until light and frothy, 1 to 2 minutes.

In a medium bowl, whisk flour, cocoa powder, and salt.

Add flour mixture, reduced Guinness, and melted chocolate mixture to egg mixture, beating just to combine after each addition.

Divide batter among prepared cups, filling about one-half full.

Bake cakes until edges are puffy and the center still jiggles a little, 7 to 9 minutes.

Transfer tin to a wire rack and let cool for 1-2 minutes.

Using a pairing knife, loosen edges. Place a parchment-lined baking sheet upside down on top of tin, then flip both over, inverting cakes onto sheet.

Using a metal spatula, quickly transfer cakes to serving plates, keeping them upside down.

Serve immediately with a dollop of whipped cream.