For the cupcakes: Preheat the oven to 350 degrees F. Line the muffin tin with cupcake liners.
Put the butter, cocoa and stout in a microwave-safe bowl, cover loosely with plastic wrap, and microwave until the butter melts, about 2 minutes. Whisk to combine.
Meanwhile, whisk the granulated sugar, flour, baking powder, baking soda and salt together in a large bowl. Whisk the sour cream, vanilla and eggs together in a small bowl.
Whisk the cocoa mixture into the dry ingredients. Stir in the sour cream mixture just until combined, being careful to avoid over-mixing. Divide the batter among the prepared muffin tins, filling each about three-quarters full, or about 1/4-cup per cupcake. Bake until a toothpick inserted in the center of the cupcakes comes out clean, about 25 minutes. Cool the cupcakes completely in the tin on a cooling rack, about 15 minutes. Remove from the liners and return the cupcakes to the rack.
For the frosting: Place the brown sugar, cinnamon, ginger, allspice, nutmeg and egg whites in a large metal mixing bowl set over a pot of simmering water. Whisk constantly until the sugar melts and the mixture is very thin and warm. Remove the bowl from the heat and transfer to a stand mixer fitted with the whisk attachment. Whisk on high speed until stiff peaks form, about 5 minutes. Continue beating on low speed until cool.
With the mixer set to medium speed, beat the butter a few pieces at a time. The mixture may curdle before coming together. Using a spoon, offset spatula or piping bag, dollop, spread or pipe about 1/2 cup of the frosting on each cupcake. Occasionally beat the frosting to maintain the smooth texture. Decorate with sugar flowers if using and serve.
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Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
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