Preheat the oven: Preheat the oven to 350°F (180°C). Grease two 9-inch (23cm) round cake pans and line the bottoms with parchment paper.
Make the cake: In a medium bowl, whisk together the flour, baking powder, and salt. In a large bowl, use an electric mixer to beat the sugar and butter until light and fluffy, about 3 minutes. Beat in the eggs one at a time, then beat in the lemon zest and juice. With the mixer on low speed, gradually add the flour mixture to the wet ingredients in three parts, alternating with the buttermilk, beginning and ending with the flour mixture. Beat just until combined.
Divide the batter: Divide the batter evenly between the prepared pans and smooth the tops.
Bake the cakes: Bake for 30-35 minutes, or until a toothpick inserted into the center of each cake comes out clean.
Make the lemon buttercream frosting: In a large bowl, use an electric mixer to beat the butter until creamy, about 2 minutes. Gradually add the powdered sugar, beating until smooth and creamy. Beat in the lemon zest and juice, then beat in the salt.
Assemble the cake: Once the cakes are completely cool, place one cake layer on a serving plate or cake stand. Spread a thick layer of lemon buttercream frosting on top of the cake. Top with the second cake layer and frost the top and sides of the cake with the remaining frosting.