Preheat the oven to 350 degrees F. Line 12 muffin cups with paper cupcake liners.
Sift the flour, baking powder and salt into a medium bowl. Set aside. Stir together the milk and vanilla in a small bowl. Set aside.
Beat the butter using an electric mixer on low speed 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, 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 using 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 among the prepared muffin cups.
Bake until a toothpick inserted in the center of the cakes comes out clean, rotating the tin halfway through, about 20 minutes. Cool the cupcakes on a rack in the tin for 10 minutes, and then remove from the tin to cool completely on the rack.
For the frosting:
Beat together the cream cheese and butter using an electric mixer on medium speed until fluffy, about 2 minutes. Beat in the vanilla and lemon juice. Reduce the mixer speed to low and gradually add the confectioners' sugar, beating until well combined.
Transfer the frosting to a piping bag or resealable bag and pipe onto the tops of 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.)