Preheat the oven: Set your oven to 350°F and line a 8x8-inch baking pan with parchment paper for easy removal.

Cream the butter and sugars: In a large bowl, mix the butter, brown sugar, and granulated sugar until light, fluffy, and well combined.

Add the wet ingredients: Mix in the eggs, vanilla extract, and salt until everything is smooth and fully incorporated.

Combine with the dry ingredients: Add the flour and stir just until the dough comes together—avoid overmixing to keep the bars soft.

Fold in the mix-ins: Gently fold in the mini chocolate chips, Christmas M&M’s, and crushed Oreos, saving a little extra for topping.

Press the base layer: Press about one-third of the cookie dough evenly into the bottom of your prepared baking pan.

Layer the whole Oreos: Arrange the 9 whole Oreos in a single, even layer over the dough.

Add the remaining dough: Spread the rest of the cookie dough over the Oreos, gently pressing to cover them completely.

Add decorative toppings: Sprinkle a few extra crushed Oreos and M&M’s on top for a festive finish.

Bake the bars: Bake for 22–25 minutes, or until the top is golden and a toothpick inserted into the center comes out mostly clean.

Cool and slice: Let the bars cool in the pan for 10–15 minutes before slicing into squares and serving.