Preheat the oven to 350°F. In a bowl, beat the softened butter, brown sugar, and white sugar until smooth.
Beat until fully incorporated. Add vanilla extract and the egg to the creamed mixture, beat again.
In a separate bowl, mix the dry ingredients: flour, baking soda, and salt. Gradually add them to the wet mixture, beating until combined.
Stir in the chocolate chips and most of the M&M’s, reserving some for topping.
Spread the dough evenly into a 8x6-inch (or 9x7) baking pan. Sprinkle the reserved M&M’s on top.
Bake in the preheated oven for 18–20 minutes, or until the surface is golden brown.
Allow the cookie bars to cool in the pan before slicing into squares.
Notes
Room Temperature: Keep them in an airtight container for up to 3 days. Layer with parchment or wax paper to prevent sticking.Refrigerator: Store in an airtight container for up to 1 week. Let them come to room temperature before serving for the best texture.Freezer (Optional): Wrap individually or in layers with parchment paper and store in an airtight container or freezer bag for up to 3 months. Thaw at room temperature before enjoying.