Heat oven to 350 degrees. Line muffin tins with paper baking cups. In a large bowl, beat cake mix, water, oil, eggs, and vanilla with electric mixer on low speed until well combined. Pour 1/3 of the batter into each baking cup. Place one mini brownie in the middle of each baking cup and continue to fill 3/4 of the way to the top, making sure the brownie is covered. Bake for 15-20 minutes, or until a toothpick inserted into the batter comes out clean. Cool completely and pipe on frosting, decorating as desired.