Preheat the oven to 350 F. Line one 12-cup standard muffin tin with paper cupcake liners.
Sift the flour, baking powder and salt together into a medium bowl. Stir the milk and vanilla together in a small bowl. Set aside.
Beat the butter on low speed in a large bowl using an electric mixer until smooth, about 1 minute. Gradually beat in 1/2 cup of the granulated sugar. Increase the speed to high and beat until light and fluffy, scraping down the sides of the bowl as needed, about 5 minutes. Reduce the mixer speed to low. Add about one-third of the flour mixture and beat until just combined. Add half the milk mixture and beat until smooth. Repeat, adding flour, then milk, and then the rest of the flour, stopping and scraping down the sides of the bowl as needed. Increase the mixer speed to medium and beat briefly to make a smooth batter. Clean the medium bowl and the mixer beaters.
Put the egg whites in the clean medium bowl. Whip on high speed with the electric mixer until foamy. Gradually add the remaining 1/4 cup granulated sugar and continue to beat until the egg whites are glossy and hold stiff peaks, about 10 minutes. Fold one-third of the whites into the batter to lighten it and then fold in the remaining whites. Divide the cake batter evenly between the liners. Clean the large bowl and the mixer beaters.
Bake until a tester inserted in the center of the cakes comes out clean, rotating the tin about halfway through, about 20 minutes. Cool the cupcakes on a rack in the tin for 10 minutes, and then remove from the tin. Cool on the rack completely.
For the frosting:
Beat the butter and confectioners' sugar in the clean large bowl with the electric mixer on medium-high speed until light and fluffy, scraping the sides of the bowl occasionally, 5 to 7 minutes. Add the marshmallow, milk, vanilla and salt and beat on low speed until blended. Increase the mixer speed to high and beat until fluffy and thick, 1 to 2 minutes. Pipe or spread the frosting on the tops of the cupcakes and top with sprinkles.
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When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)