Preheat 350 degrees.
Spray {or butter} a 9 inch round cake pan or pie plate with non stick spray.
Cream butter and brown sugar together in a large mixing bowl on medium speed.
Add egg and egg yolk then vanilla.
In a separate large bowl add flour, cornstarch, baking soda, and salt. Mix well.
With the mixer on low speed, slowly add the flour mixture until fully combined. The dough will be thick.
Add the chopped candy and Oreos, reserving 2 tbsp M&M and crushed Oreos for topping.
Gently stir until combined then spread evenly into the pan.
Press remaining 2 tbsp M&M's on top of the cake.
Bake for 18-20 minutes until cooked through.
Cool slightly then remove onto a cooking rack to cool completely.
Optional: once cooled add decorative frosting around the edges and sprinkle with crushed Oreos.
Slice and serve and enjoy this Chocolate Lovers Cookie Cake!