Step 1: Preheat the Oven

1. Set the Temperature: Preheat your oven to the temperature specified on the brownie mix package, usually around 350°F (175°C).

Step 2: Prepare the Baking Pan

1. Grease the Pan: Lightly grease an 8×8-inch (20x20cm) or 9×9-inch (23x23cm) baking pan with butter or cooking spray.
2. Line with Parchment Paper: For easy removal, line the pan with parchment paper, leaving an overhang on the sides.

Step 3: Mix the Ingredients

1. Combine Wet Ingredients: In a large mixing bowl, whisk together the Greek yogurt, water, and vegetable oil until smooth. If using vanilla extract, add it here.
2. Add the Brownie Mix: Pour the brownie mix into the bowl and stir until just combined. Avoid overmixing to prevent dense brownies.
3. Fold in Add-Ins: If using chocolate chips or nuts, gently fold them into the batter.

Step 4: Bake the Brownies

1. Transfer to Pan: Pour the batter into the prepared baking pan, spreading it evenly with a spatula.
2. Bake: Place the pan in the preheated oven and bake for the time specified on the brownie mix package, usually 25-30 minutes. Check for doneness by inserting a toothpick into the center—it should come out with a few moist crumbs but not wet batter.

Step 5: Cool and Serve

1. Cool in Pan: Let the brownies cool in the pan for 10-15 minutes.
2. Remove and Slice: Use the parchment paper overhang to lift the brownies out of the pan. Transfer them to a wire rack to cool completely before slicing.