Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
In a large bowl, beat the softened butter until creamy.
Gradually add the powdered sugar and cocoa powder, beating until well combined.
Add the buttermilk, vanilla extract, and salt. Beat until the frosting is smooth and fluffy.
Once the cakes are completely cooled, spread a layer of frosting on top of one cake layer. Place the second cake layer on top and frost the top and sides of the cake.
