Preheat oven to 350 degrees (F). Lightly spray a 9" pie plate with non-stick baking spray; set aside.
In a large bowl add the melted butter, brown sugar, and granulated sugar; beat well until combined. Cool for 5 minutes, then beat in the egg, egg yolks, and vanilla. Add in the flour, cornstarch, and salt and mix until just combined. Be sure not to over mix here! Fold in the chopped candy bars. Spread the mixture into the prepared pie plate (it will be very thick), smooth the top, and bake for 35 minutes, or until the top is golden brown, the edges are completely set, and the center of the pie is only slightly jiggly. Top the warm cookie cake with extra candy, if desired. Transfer cookie cake to a wire rack to cool for at least one hour before topping with frosting (if using) and slicing.
For the chocolate frosting:
Sift together the confectioners sugar and cocoa powder, whisking well to ensure it's bump free.
Using a handheld mixer OR a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until creamy; about 2 minutes. Reduce speed to low and slowly add the sifted sugar/cocoa powder, alternating with the cream, then add the vanilla and salt. Once all of the ingredients have been added, beat on high speed until light and creamy, about 2 minutes.
Scrape frosting into a piping bag fitted with an open star tip and pipe onto the cooled cake.
You can use any variety of chocolate candy you have on hand for this cake. Cake is best eaten the day it's made, but will keep, covered, for 3 days.