Preheat oven to 325. Prepare a baking sheet with parchment paper and set aside. In a medium-sized bowl or zip-top bag whisk together all dry ingredients: flour, cocoa powder, baking powder, and salt; set aside.
In a large, microwave safe bowl, combine the chocolate chips and butter. Heat them in the microwave on low-power for 20 second increments. Mix in between each until butter and chocolate are melted and whisk until smooth. Whisk in the vanilla, then whisk in both sugars. Beat in eggs, one at a time. Add the milk and beat until incorporated.
Add the dry ingredients into the wet ingredients, and stir to combine with a spatula so you don’t overmix. Fold in Reese’s Pieces and chocolate chips, mix together until just combined.
Grease a large cookie scoop or 1/4 cup, scoop cookie dough, then insert one dollop of frozen peanut butter mixture into the center each cookie dough ball, forming the chocolate dough around the peanut butter dollop until smooth and the mound shape remains. Place onto the prepared baking sheet. *Batter will be sticky*
Bake for 5 minutes (half-baked), remove cookie tray from the oven and use a spatula or bottom of a measuring cup to lightly press the cookie dough down to help flatten. Then, press extra Reese’s Pieces and chocolate chips onto the tops of the cookies. Finish baking for another 5 minutes or so until they are just set but a little gooey in the middle.
Allow cookies to cool on the tray for 10 minutes before removing.