Step 1
Preheat oven to 350 degrees F (175 degrees C). Line a 9-inch baking pan with parchment paper; grease parchment paper with cooking spray.
Step 2
Whisk flour sugar baking powder baking soda and salt together in a bowl.
Step 3
Combine buttermilk egg butter and vanilla extract together in a small bowl beating to break up the egg. Add flour mixture; stir until just combined. Fold chocolate chips gently into the batter. Allow batter to rest for 5 minutes.
Step 4
Combine brownie mix oil eggs and water in a large bowl; whisk together until smooth.
Step 5
Pour 1/2 cup of the brownie batter into a squeeze bottle pastry bag with a small round tip or large resealable plastic bag with corner snipped off. Reserve remaining batter.
Step 6
Heat a large nonstick skillet over medium-low heat. Drop 1/3 cup pancake batter into the skillet shaking skillet to spread batter evenly. Drop 5 nickel-size dollops of brownie batter over the pancake; swirl using a skewer or knife edge. Cook until edges are dry 2 to 3 minutes. Flip and cook until set on the other side 1 to 2 minutes. Repeat with remaining batters.
Step 7
Pour reserved brownie batter into the prepared pan.
Step 8
Bake in the preheated oven until a toothpick inserted into the center comes out clean 28 to 30 minutes. Cool on a wire rack before slicing.