Preheat oven to 350° Spray a 9" round springform pan (that's what I used), baking pan or pie dish very well with non-stick spray and set aside.
In a large mixing bowl, mix melted butter and both sugars together until smooth. Add in eggs and vanilla and mix until smooth. Pour in the flour and cornstarch and mix until just combined - you don't want to over mix.
For the mix-ins, I reserved about 2 tablespoons of each to press into the top of cookie cake before baking, but that's optional.
Fold in the chopped hershey bar, chocolate chips, twix and M&M's and mix until evenly distributed. Turn the dough out into prepared pan and smooth until even and level. Press remaining candy into top of cookie cake where ever you desire.
Bake for about 20-23 minutes or until toothpick inserted in center comes out clean or with a few moist crumbs. This creates a very soft, under baked cookie center with chewy edges, but if you like your cookies a little more well-done, you can bake for longer. Allow to cool cool for about 20 minutes before slicing and serving so the chocolate has time to set a little.
Cookie cake will stay fresh for up to 7 days in an airtight container.