Preheat the oven to 350 (F) -- this is a 25 degree (F) increase from the original sea-level recipe temperature of 325 (F).
Put 3/4 plus 2 tablespoons flour, 2 1/2 tablespoons cocoa powder, 3/4 teaspoons baking powder, 3/4 cup sugar, pinch of salt, and 3 tablespoons butter in a freestanding electric mixer with a paddle attachment (or use a handheld electric whisk) and beat on slow speed until you get a sandy consistency and everything is combined.
Slowly pour 1/2 cup milk into the flour mixture, beating well until all the ingredients are well mixed.
Add the egg and beat well, scraping any unmixed ingredients from the side of the bowl with a rubber spatula.
Spoon the batter into the paper cases until two-thirds full and bake in the preheated oven for about 20 minutes, or until the cake bounces back when touched. Let the cupcakes cool slightly in the pan before turning out onto a wire rack to cool completely.