Preheat the oven to 375°F. Line three sheet trays with parchment paper and set aside.
Cream together the butter, brown sugar, and granulated sugar in the body of a stand mixer with the paddle attachment until smooth and combined, 2 minutes. Scrape down the sides.
Add the eggs and vanilla, mix in until combined.
Add the flour, cocoa powder, cornstarch, salt, and baking soda, mix it in well until combined, scrape down the sides as needed.
Portion the cookie dough into 6-ounce cookie dough balls by weight.
Layout a piece of parchment paper and place one of the portions on top. Using your hands or a rolling pin, form the cookie dough into a circle about 1/4th inch thick. I find placing another piece of parchment paper on top helps to spread it out without sticking.
Take a knife and make a slice halfway through the cookie dough, making a line from the center of the cookie circle to the bottom edge. You will now have four sections to add the fillings.
In the bottom left corner add 1/5th of the Reese’s pieces. In the upper left corner, add 1/5th of the Reese’s peanut butter chips. In the upper right corner, add 3 Reese’s cups thins. In the bottom right corner, add 3 more Reese’s cups thins.
Carefully fold the bottom left corner up and over the top left corner, so the pieces will now be on top of the peanut butter chips.
Now, fold over the top left triangle over the top right.
Lastly, fold the top right over the bottom right corner. You will now have a triangle. Pinch the seams together. If desired, form it into a circle or you can leave it in more of a triangle shape.
Place the cookie on the sheet tray. Repeat with the remaining dough and fillings.
The cookies are so big only two can bake at a time on the sheet tray. If desired add more Reese’s pieces to the tops of the cookies.
Bake for 18-23 minutes until golden brown along the edges and the tops are no longer glossy.
Let the cookies cool on the sheet tray for 10 minutes then transfer to a wire rack to cool completely.