Preheat and Prepare Pan
Preheat oven to 350°F (175°C). Spray a 9×13-inch baking pan with non-stick spray or line with parchment paper, leaving overhang for easy removal.

Mix Wet Ingredients
In a large bowl, whisk together melted butter, eggs, water, and vanilla extract until smooth.

Add Cake Mix
Stir in the white cake mix until just combined. The batter will be thick. Do not overmix.

Fold in Chocolate Chips
Gently fold in mini chocolate chips until evenly distributed.

Spread Batter
Transfer batter to prepared pan and spread evenly into corners using a spatula.

Add M&Ms
Sprinkle 1 cup of Easter M&Ms evenly over the batter. Press lightly so they stick.

Bake
Bake for 18–22 minutes, until edges are lightly golden and a toothpick inserted in the center comes out clean or with a few moist crumbs. Avoid overbaking.

Top and Cool
Immediately sprinkle the remaining 2–3 tablespoons of M&Ms on top. Gently press them into the warm bars. Cool completely in the pan on a wire rack.

Slice and Serve
Once fully cooled, lift bars out using parchment overhang (if used). Cut into 15–24 squares using a sharp knife, wiping clean between cuts for neat edges.