In a large mixing bowl, beat butter and brown sugar until creamy.
Add egg, egg yolk, and vanilla extract, beating until combined.
In a medium bowl, whisk together flour, cornstarch, baking soda, and salt.
Add the dry ingredients to the mixing bowl, and beat until combined.
Fold in semi-sweet chocolate chips, mini chips, and M&M'S® Caramel.
Spray an 8-inch springform pan with non-stick spray.
Spread batter evenly into the bottom of the springform.
Bake at 350 degrees for 30 minutes, or until the edges and top are golden, and a toothpick inserted near the center comes out clean.
Remove to a wire rack to cool.
Once cool, run a knife around the edge to loosen the cookie. Release the springform pan, and transfer the cookie to a serving plate.