Preheat oven to 350°.
In the bowl of a stand mixer, add the melted butter, brown sugar and granulated sugar.
Turn the mixer onto low and combine the butter and sugars.
Leave the mixer going and add in the eggs, corn syrup and vanilla.
Turn the mixer off and add in the flour, cocoa, corn starch, salt and soda. Mix until just combined. The dough will be very soft and slightly sticky.
Mix in the chocolate and peanut butter mix ins until evenly distributed.
Scoop the cookie dough into equal portions. For small cookies, use 1 1/2 tbsp of cookie dough (#50 scoop), for larger cookies use 3 tbsp of cookie dough (#24 scoop). Roll into a ball and then flatten slightly.
Bake at 350° F until the cookies spread slightly and the tops no longer look wet and glossy. For small cookies, bake for 7-9 minutes. For the larger cookies, bake for 9-11 minutes.
Allow the cookies to cool on the hot cookie sheet for 10 minutes before transferring to a wire rack.
Store in an air tight container for up to 3 days- the cookies are best eaten same day, because the pretzels and peanuts will get soft.