Preheat oven to 350°F (175°C) and position the rack in the center.
Grease two 9‑inch round cake pans, line the bottoms with parchment, and lightly flour the sides.
Sift together flour, baking powder, and salt; set aside.
Cream butter and sugar together on medium speed until pale and fluffy, about 3‑4 minutes.
Add eggs one at a time, beating well after each addition, then mix in vanilla extract.
With the mixer on low, alternately add the dry ingredients and milk, beginning and ending with the dry mixture. Mix just until combined.
Divide batter evenly between the prepared pans and smooth the tops.
Bake for 30‑35 minutes, or until a toothpick inserted in the center comes out clean; rotate pans halfway through baking.
Cool cakes in pans for 10 minutes, then turn out onto a wire rack to cool completely.
For the buttercream, beat softened butter until creamy, then gradually add powdered sugar, salt, milk, and vanilla, beating until light and airy.
Level the tops of the cooled cakes with a serrated knife, then stack with a layer of buttercream between them.
Frost the top and sides of the cake evenly, smoothing with an offset spatula.
Decorate as desired and serve.