Get the recipe at Big City, Little Kitchen, which says:
Despite the addition of Guinness, these cakes are not at all bitter; instead, the beer adds richness and moisture, and balances the sweetness of the sugar. Working from another Nigella recipe, this one the Guinness Cake from Feast, I substituted brown sugar for white to add depth of flavor, and made tiny cakes instead of a large one in a springform. Topped with a cream cheese glaze, these are a crowd-pleasing, not-too-sweet dessert (and, if you call them muffins instead of cupcakes, and play up the beer angle, you can easily pass them off to guys). They’re also super-easy.