Inspired by the Italian cocktail, this cupcake evokes the herbal and citrus flavors of a classic Negroni. We added Campari to the icing and have scented the white cupcake with grapefruit, cardamom, and pink peppercorns to evoke a sweet and slightly bitter flavor profile.
Make the cupcakes: Preheat the oven to 350˚ F. Line a 12-cup muffin tin with foil or paper liners. Whisk the flour, baking powder, salt, cardamom and both peppers in a medium bowl. In a large bowl, beat the butter, granulated sugar and grapefruit zest with a mixer on medium-high speed until light and fluffy, about 3 minutes. Reduce the speed to medium low and beat in the flour mixture in three additions, alternating with the milk in two additions, until just combined.
In a clean bowl with clean beaters, beat the egg whites on high speed until they hold stiff peaks but are not dry and clumpy, about 4 minutes. Gently fold the egg whites into the batter until no white streaks remain. Divide the batter evenly among the muffin cups. Bake until the cupcakes are golden and a toothpick inserted into the centers comes out clean, 22 to 24 minutes. Let cool 5 minutes in the pan, then remove to a rack to cool completely.
Meanwhile, make the frosting: Beat the butter and confectioners' sugar in a large bowl with a mixer on medium speed until combined, then increase the speed to medium high and beat until smooth and fluffy, about 3 minutes. Beat in the Campari (or grapefruit juice), then tint with red and orange food coloring until Campari-colored. Spread the frosting on the cupcakes with an offset spatula. Sprinkle with the coarse sugar and garnish each with an orange slice.