Preheat oven to 350 degrees F and line a rimmed baking sheet with a silicone baking mat or parchment paper.
In a large bowl, beat together butter, brown sugar, and egg yolk until combined using a hand mixer.
Beat in maple syrup and vanilla extract until well combined.
Add in flour, cocoa powder, baking soda, and salt. Beat together until fully combined.
Beat in cadbury eggs and m&ms. Cover and chill dough for 30 minutes, if desired (they spread more if you don’t chill, but I always skip this step).
Using a large cookie scoop, scoop 9 cookies onto the prepared baking sheet.
Bake in preheated oven for 10-15 minutes (I did 12), or until cookies are cooked to your liking.
Immediately, gently press extra M&Ms & Cadbury Eggs into the tops of each cookie. This is just to make them pretty.
Let cool for a few minutes on the baking sheet, then remove, and finish cooling on wire rack.
Repeat with remaining cookie dough.