Preheat oven to 325°F. Grease a 9x13-inch baking pan or line it with parchment paper.

In a large bowl, beat the softened butter, brown sugar, and granulated sugar until creamy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Fold in 1 cup of M&M's candies and 3/4 cup of semi-sweet chocolate chips.

Spread the dough evenly into the prepared baking pan.

Sprinkle the remaining M&M's candies and chocolate chips over the top.

Bake for 25 to 35 minutes or until golden brown and a toothpick inserted into the center comes out clean.

Allow the cookie bars to cool completely in the pan before cutting into squares.