Preheat the oven to 350 degrees F (175 degrees C). Line 48 muffin cups with paper liners.
Make brownie batter: Whisk together brownie mix, eggs, oil, and water in a large bowl until fully incorporated, about 50 strokes. Set aside.
Make cake batter: Beat cake mix, water, egg whites, and oil in a separate bowl with an electric mixer on low speed until combined. Increase speed to medium and beat for 2 minutes.
Spoon brownie batter into the prepared muffin cups, about 1/3 full. Spoon cake batter over brownie batter until each muffin cup is about 2/3 full.
Bake in the preheated oven until golden and the tops spring back when lightly pressed, about 20 minutes.