Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line the bottoms with parchment paper and grease the sides.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until well combined. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar on medium-high speed until light, fluffy, and pale in color, about 3-5 minutes. Scrape down the sides of the bowl as needed.
Step 4: Beat in the eggs one at a time, mixing well after each addition until fully incorporated. Stir in the pure vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk. Begin and end with the dry ingredients. Mix on low speed until just combined after each addition. Be careful not to overmix the batter.
Step 6: Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula.
Step 7: Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cakes comes out clean. The cakes should be lightly golden on top.
Step 8: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure cakes are fully cooled before frosting.
Step 9: While cakes cool, prepare the buttercream: In a large bowl, beat softened butter until creamy. Gradually add sifted powdered sugar, milk/cream, vanilla, and salt. Beat on high speed until light and fluffy. Frost your cooled cake layers and serve.