Step 1: Preheat and Prep

Begin by preheating your oven to 350°F (175°C). This is crucial to ensure even baking and prevent the bars from being undercooked in the middle or overcooked on the edges. While the oven is preheating, prepare a 9x13 inch baking pan. Lightly grease the pan with non-stick cooking spray to prevent the bars from sticking. Alternatively, you can line the pan with parchment paper, leaving an overhang on the sides. This will make it easier to lift the bars out of the pan once they've cooled, resulting in clean, even slices. Ensure the cooking spray or parchment paper covers the entire bottom and sides of the pan.

Step 2: Combine Ingredients

In a large mixing bowl, combine the yellow cake mix, eggs, vegetable oil, and vanilla extract. Use an electric mixer (handheld or stand mixer) on medium speed to blend the ingredients until they are fully incorporated. Be careful not to overmix, as this can result in tough cookie bars. Mix until just combined, ensuring there are no large lumps of cake mix remaining. Scrape down the sides of the bowl with a spatula to ensure all ingredients are evenly mixed.

Step 3: Add the Mix-Ins

Once the base mixture is smooth, gently fold in the Easter M&M candies and semi-sweet chocolate chips. Folding is a technique that involves gently incorporating the ingredients without deflating the batter. Use a spatula to carefully combine the candies and chocolate chips into the batter, ensuring they are evenly distributed throughout. This step adds texture, flavor, and visual appeal to the cookie bars. You can substitute other types of chocolate chips, such as milk chocolate, dark chocolate, or white chocolate, depending on your preference. Similarly, you can use other seasonal candies or nuts in place of the M&M's.

Step 4: Spread into Pan

Pour the cookie dough into the prepared 9x13 inch baking pan. Use a spatula or your fingers to spread the dough evenly across the bottom of the pan. The dough will be thick, so you may need to apply gentle pressure to ensure it reaches all corners. Try to create a smooth, even surface to ensure the bars bake uniformly. If desired, you can sprinkle a few extra M&M's and chocolate chips on top of the dough for added visual appeal.

Step 5: Bake and Cool

Place the baking pan in the preheated oven and bake for 20-25 minutes, or until the edges of the bars are golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs. The baking time may vary depending on your oven, so it's important to check the bars periodically. Once the bars are done, remove them from the oven and place them on a wire rack to cool completely. Allowing the bars to cool completely is crucial for achieving the best texture and preventing them from crumbling when sliced. This usually takes about 1-2 hours. Resist the temptation to cut into them while they're still warm!