Preheat your oven to 350°F (175°C). Line your 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and well combined. Add in the room temperature egg and vanilla extract, whisking until fully incorporated.
In a separate bowl, sift together the all-purpose flour, cocoa powder, baking soda, salt, and corn starch. This helps to aerate the dry ingredients and prevents clumping.
Gradually add the dry mixture to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix, as this can lead to tough cookie bars.
Carefully fold in the semi-sweet chocolate chips and chocolate M&M's, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, smoothing the top with your spatula. Bake in the preheated oven for about 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the pan from the oven and allow the cookie bars to cool in the pan for about 10 minutes. Use the parchment paper overhang to lift them out of the pan and transfer to a wire rack to cool completely. Once cool, slice into squares and serve!
