Preheat the oven to 350º F. Grease two 8 inch springform pans with butter (butter the sides and bottom) and line each pan with a parchment paper circle for easy cake removal. Set aside. 

In a large mixing bowl combine the cake mix, flour, sugar, salt, eggs, vanilla extract, milk, buttermilk, and sour cream, beat with an electric mixer until combined, 1-2 minutes.

Evenly divide the batter between the 2 pans, with 3 & 1/3 cups of batter in each pan. If using bake even cake strips, secure them around the outside of the pans. 

Bake both pans on the same rack in the center of the oven for 35-40 minutes or until a toothpick inserted in the center comes out clean. Baking times will vary depending on your oven and size of pan you’re using. 

Allow the cakes to cool for 20-30 minutes in the pans before removing them and allowing them to cool completely on a cooling rack. 

In a mixing bowl, beat the butter on high for 3 minutes. 

Sift in the cocoa powder and 3 cups of powdered sugar. I like to sift these in because it helps break up clumps, creating a light and fluffy frosting. Beat the frosting on low speed until combined and thick.

Mix in 3 tbsp of heavy cream, vanilla extract and salt. 

Add in the additional powdered sugar (1-2 cups), mix on low. If the frosting is too thick, beat in an additional tablespoon of heavy cream. 

If needed, level the cooled cakes using a cake leveler or serrated knife. Spread a small amount of chocolate buttercream on the bottom of your cake plate / stand, this acts as “glue” to prevent the cake from moving. Place one cake layer over the frosting. Measure 3/4 to 1 cup of frosting and evenly spread that on top of the first layer. Top with the second cake layer and spread the remaining frosting around the sides and top of the cake. I use about 1 cup of frosting on the top of the cake. Top with chocolate curls if desired. 

Slice and serve. Cover leftover cake tightly with plastic wrap and store on the counter for up to 3 days.