• Preheat oven to 350 degrees F. Grease and flour two 9x2 inch round cake pans
• Combine cake mix, flour, sugar, salt, eggs, vanilla, oil, butter and milk in a large bowl. Beat with an electric mixer on medium speed until smooth (about 2 minutes).
• In a small mixing bowl, mix together cocoa powder and hot liquid. Add sour cream and brown sugar and stir until smooth. Gradually stir in ¾ cup (180 grams) of the batter, stirring until smooth to make a chocolate batter.
• Divide most of the remaining vanilla batter evenly among pans. Drop gobs of chocolate batter over the vanilla batter, then scrape any remaining vanilla batter off the walls of the bowl and dot around. Swirl through.
• Bake for 25 to 30 minutes on center rack or until cakes test done. Let cool in pans for 10 minutes, then carefully invert. Let cool completely.

Frosting

• To make the frosting, put the butter in the bowl of a stand mixer and beat with the paddle until creamy. Add the powdered sugar and beat until it comes together with the butter, then add the cocoa powder, vanilla, salt and 2-3 tablespoons of the milk.
• Beat until everything is blended together but still a little lumpy. Scrape the sides of the bowl, increase mixer speed and beat until creamy, adding the remaining milk as needed.