Special equipment: Two 8-inch round cake pans
For the cake: Preheat the oven to 350 degrees F. Grease the bottom of two 8-inch round cake pans and line with waxed paper or parchment paper.
Sift together the flour, salt and baking powder, then set aside. In a large bowl using an electric mixer, cream the granulated sugar and butter together until light and fluffy. Add the vanilla and eggs, and beat until thoroughly mixed. Alternate adding the flour mixture and the milk, a little at a time, beating well after each addition. Continue beating for 1 minute. Spread the batter evenly into the prepared pans. Bake until a toothpick inserted in the center comes out clean, 30 to 35 minutes.
Cool the cakes in the pans on a wire rack for 10 minutes. Remove the cakes from the pans and cool completely on the wire rack.
For the buttercream: In a stand mixer fitted with a whip attachment, beat the butter with the vanilla and salt. A little at a time, mix in 5 cups of the confectioners' sugar and the milk and whip until smooth, adding more milk or up to 1 cup more confectioners' sugar if needed.
Frost the top of one cake and place the second cake on top. Then spread the frosting over the top and sides of the cake. Serve at room temperature.
Use the buttercream immediately or store in an airtight container at room temperature for 24 hours or in the refrigerator for up to 2 weeks.