Preheat the oven to 350 degrees F (175 degrees C). Line two 12-cup muffin tins with paper liners.
Mix flour, baking powder, and salt in a medium bowl.
Beat butter and sugar together in a large bowl using an electric mixer until creamy. Add eggs, 1 at a time, beating well after each addition. Mix in vanilla extract. Add the flour mixer and milk alternately. Fill liners 2/3 full with the batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 15 to 23 minutes.
Bring a pan of water to simmer. Combine sugar, egg whites, and salt in the bowl of a stand mixer. Place bowl on top of the pan; whisk until sugar dissolves.
Attach bowl to the stand mixer fitted with a whisk attachment. Beat until stiff peaks form and mixture is cool and glossy. Add butter a few tablespoons at a time, beating well after each addition. Add vanilla and mix until buttercream is smooth.
Cool cupcakes in the tins for 5 minutes. Transfer to a wire rack and let cool, about 20 minutes. Frost cupcakes with the buttercream.
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