Preheat the oven to 350 degrees F. Line a 12-cup standard muffin tin with paper cupcake liners.
Sift the flour, baking powder and salt into a medium bowl. Stir the milk and vanilla together in a small bowl. Set aside.
Beat the butter in a large bowl using an electric mixer on low speed until smooth, about 1 minute. Gradually beat in 1/2 cup of the sugar. Increase the speed to high and beat until light and fluffy, about 5 minutes. Scrape down the sides of the bowl as needed.
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 remaining 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; set aside.
Put the egg whites in a clean bowl. Whip with an electric mixer on high speed until foamy. Gradually add the remaining 1/4 cup 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 cups of the prepared muffin tin.
Bake, rotating the tin halfway through, until a tester inserted in the center of the cakes comes out clean, about 20 minutes. Cool the cupcakes on a rack in the tin for 10 minutes, then remove from the tin and cool completely on the rack.
For the frosting:
Bring a few inches of water to a boil in a saucepan. Whisk together the egg whites, sugar, lemon juice, cream of tartar and salt in a medium heat-safe bowl. Set the bowl over the saucepan, making sure the bottom of the bowl does not touch the water. Continue whisking until the mixture is hot to the touch and the sugar dissolves, about 2 minutes. Remove the bowl from the water and whip with an electric mixer at medium-high speed until the meringue is cool and holds a soft peak, about 5 minutes. Add a few drops of desired food coloring to tint the frosting and continue beating until it holds stiff peaks.
Spoon or pipe the frosting decoratively and generously on the cupcakes.
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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.)