One 18-ounce box brownie mix (plus required ingredients)
1/2 cup sour cream
2 tablespoons light brown sugar
1/2 teaspoon cinnamon sugar
2 drops vanilla extract
Prepare the brownie batter according to the package instructions, then transfer to a baking dish. Do not bake.
In a small bowl, stir together the sour cream, brown sugar, cinnamon sugar and vanilla. Drop the mix randomly around the top of the batter. Drag a skewer through the sour cream areas and batter areas to make random streaks and swirls.
Bake and cool according to the package instructions.