Using an electric mixer, beat butter and sugar together on medium-speed until light and fluffy, about 2 minutes. Add eggs and vanilla and beat to combine. Stir together the flour, cocoa, baking soda, cornstarch, and salt in a separate bowl until combined. Add the dry ingredient mixture to the wet ingredients, and beat on medium-low speed until just combined. Carefully fold in the peanut butter cups by hand until combined.
Turn the dough into a 9-inch* skillet that has been well-greased (I used butter) on all sides. Press the dough into an even layer in the skillet, leaving a 1/2-inch border around the edges so that the dough is not actually touching the sides of the pan.
Bake for 15-18 minutes, or until a toothpick inserted in the center of the cookie comes out clean. Let cool for at least 5 minutes on a wire rack. When ready to eat, top with ice cream and sprinkle on additional peanut butter cups. Then melt some chocolate and peanut butter in separate bowls in the microwave, and drizzle on top.