How to store M&M cookie bars: Keep leftover cookie bars in an airtight container at room temperature for up to 4 days. You can also store the in the fridge for up to one week.

How to freeze cookie bars: These M&M chocolate chip cookie bars freeze really well. Let them cool completely first, then slice and store in an airtight container or freezer bag for up to 3 months.

Make ahead instructions: You can make the cookie dough up to 2-3 days in advance. Cover the bowl with plastic cling wrap when storing. When ready to bake, allow the dough to come to room temperature (about 20 minutes) before spreading onto the prepared baking pan.

How to freeze cookie dough: You can also freeze the cookie dough by rolling it into a ball and wrapping tightly in plastic cling wrap, and place inside a freezer bag. Freeze for up to 3 months. When ready to bake, allow the cookie dough to thaw overnight in the refrigerator, and allow it to come to room temperature before baking, according to recipe instructions.

For thinner bars: Spread the cookie dough into a lined quarter sheet baking pan or 9x13-inch pan to make thinner cookie bars. You will need to shorten the cook time though, so keep an eye on the cookies at the 15-20 minute mark.

Substitutions: If you are feeling creative, you can add in nuts, make simple chocolate chip cookie bars by omitting the M&M's, substitute the chocolate chip for white chocolate chips, and switch up the colors of M&M's based on the holiday that you are baking them for.

How to make a bigger batch: You can simply double the recipe and spread the cookie dough into a 9x13-inch baking pan.