Preheat your oven to 350°F (175°C). Gather your baking supplies and prepare your 9x13-inch baking dish.

Line the prepared baking dish with foil and spray with nonstick cooking spray.

In a stand mixer, beat softened butter with both granulated and brown sugars until light and fluffy, about 2 to 3 minutes.

Add in the eggs and vanilla extract, mixing until well incorporated, about 1 minute.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually mix this dry mixture into the wet ingredients until just combined.

Fold in the M&M'S and mini chocolate chips, reserving some for topping.

Spread the dough evenly in the prepared baking dish and press down the reserved M&M'S and chocolate chips on top.

Bake in the preheated oven for 30 to 35 minutes until golden brown and a toothpick comes out clean.

Let the cookie bars cool completely in the dish before lifting them out and slicing into squares.