In a large bowl or the bowl of a stand mixer, combine the melted butter, granulated sugar, brown sugar and vanilla. Mix on low until combined. With the mixer on low, add the eggs one at a time and mix until incorporated.
Add the flour, cocoa powder, cornstarch, baking soda and kosher salt. Mix just until a chocolate cookie dough forms.
Scoop 20 even cookie dough balls. Take each dough ball and flatten it slightly. Place one frozen peanut butter filling portion in the center. Bring the chocolate cookie dough up and completely around the peanut butter filling. Pinch any seams closed and gently roll into a ball.
Pour the mini M&Ms into a shallow dish. Press the top of the cookie dough into the mini M&M’s, then place on a parchment lined cookie sheet. Repeat with the remaining cookie dough and peanut butter filling. Cover and chill the assembled cookies in the refrigerator for 30 minutes.
Preheat the oven to 350°F. Arrange 4-5 of the chilled cookies on a parchment lined cookie sheet, leaving plenty of room between each one. Keep the remaining cookie dough refrigerated while each batch bakes.
Bake for 10-11 minutes, or until the edges are just set and the centers still look soft. Immediately after removing the cookies from the oven, use a large round cookie cutter or glass to gently reshape them if needed.
Allow the cookies to cool on the baking sheet for at least 5 minutes before carefully transferring them to a cooling rack.