Preheat the oven to 375 (F); for sea-level, decrease the oven temperature to 325 (F).
Put the flour, sugar, baking powder, salt, and butter in a freestanding electric mixer with a paddle attachment (or use a handheld whisk) and beat on slow speed until you get a sandy consistency and everything is combined.
Gradually pour in half the milk and beat until the milk is just incorporated.
Whisk the egg, vanilla, and remaining milk together in a separate bowl for a few seconds, and then pour into the flour mixture and continue beating until just incorporated.
Scrape any unmixed ingredients from the side of the bowl with a rubber spatula and continue mixing for a couple more minutes until the batter is smooth. Do not overmix.
Spoon the batter into the paper cases until two-thirds full and bake in the preheated oven for 20-25 minutes, or until light golden and the cake bounces back when touched. A skewer inserted in the center of each cupcake should come out clean.
Let the cupcakes cool slightly in the pan before turning out into a wire rack to cool completely.
When the cupcakes are cold, spoon the vanilla frosting on top and decorate accordingly.