Preheat the oven to 350°F (175°C). Line two 12-cup muffin tins with paper liners.

Mix flour, baking powder, and salt together in a medium bowl.

Beat butter and sugar with an electric mixer until creamy. Add eggs one at a time followed by vanilla, beating well after each addition. Mix in the flour mixture alternately with milk until smooth. Spoon batter into prepared muffin cups (2/3 full).

Bake in preheated oven until a toothpick inserted in the center comes out clean, about 15-23 minutes.

Cool cupcakes in the tins for 5 minutes then transfer to a wire rack to cool completely for about 20 minutes.

Bring a pan of water to a simmer. Combine sugar, egg whites, and salt in the bowl of a stand mixer. Place the bowl over the pan and whisk until sugar dissolves.

Attach the bowl to a stand mixer and beat with the whisk attachment until the mixture is cool, glossy, and stiff peaks form. Gradually add butter (a few tablespoons at a time), beating well after each addition. Mix in vanilla extract.