Step 1

Whisk together flour, cornstarch, baking soda, and salt in a medium bowl.

Step 2

In a large mixing bowl, beat softened butter and cream cheese with a hand mixer on medium speed until smooth. Beat in egg and vanilla until combined, add both sugars and beat until creamy.

Step 3

Reduce speed to low and gradually add flour mixture just until it all comes together.

Step 4

Use a rubber spatula to fold in mini chocolate chips and white chocolate chips.

Step 5

Spray a 9-inch round removable-bottom tart pan with nonstick spray and press dough evenly into the pan with your hands and a sprayed offset spatula.

Step 6

Once smooth and even thickness all around,  top with remaining white chocolate chips and m&m candies.  Gently press them into the dough. Cover and chill in the refrigerator for 1 hour.

Step 7

Preheat oven to 350°.

Step 8

Bake until golden and a toothpick inserted in the center comes out mostly clean, about 15 minutes. The toothpick will not be 100% clean like a cake would be and the cookie will continue to cook a bit in the pan once removed from the oven.

Step 9

Cool in pan on a wire rack for at least 45 minutes. Press the bottom insert of the tart pan up to release the cookie and slide it onto a plate or cutting board.

Step 10

Slice into 12 equal wedges.

Step 11

The cookies can be stored on a plate wrapped in foil or another air-tight container for up to 2 days.