Preheat oven to 350 degrees and line a jellyroll (15” x 10”) pan with parchment paper. Spray pan with non-stick spray; set aside.
In a large bowl, combine brownie mix and flour. In a saucepan, melt butter then add 4 tablespoons of cocoa powder and whisk to combine. Add boiling water and allow mixture boil for 30 seconds. Remove pan from heat. Pour butter mixture into dry brownie mix and stir slightly.
In a small bowl, combine buttermilk, eggs, baking soda and vanilla. Whisk buttermilk mixture into brownie mix and stir until smooth. Pour into prepared pan and bake for 20-25 minutes, until a toothpick inserted into the center of the cake comes out clean. Remove from oven and cool 10 minutes.
To make the frosting, melt butter in a saucepan. Whisk in cocoa powder and remove from heat. Add the dry frosting mix, milk and vanilla and whisk together until smooth. Pour mixture over the warm cake. Refrigerate cake 10-15 minutes to set frosting. Sprinkle with sea salt, then cut into squares and enjoy!