Preheat oven to 350ºF (or 176ºC). Grease an 8x8in or 7x11in nonstick baking pan* with cooking spray, then line with parchment paper. Set aside.
Place mini eggs in a ziploc bag (be sure to let out all the air inside before sealing). Crush with a rolling pin into large chunks. Set aside.
Microwave butter for 30-second intervals at 50% power until melted and hot. Immediately pour into a mixing bowl. Combine both white and brown sugars with butter, then whisk vigorously for 1 minute.
Add the eggs (separating one yolk from a third egg) and vanilla. Beat well with whisk for another minute.
Sift in flour, cocoa powder, and salt. Very gently fold into the batter mixture, until not quite fully mixed (we'll finish mixing in the next step).
Add in most of the crushed Cadbury mini eggs (about 1/2 cup) and the shredded coconut. Fold into the batter until just combined and batter looks uniform. (Do not over-mix).
Pour batter into your lined baking pan, then smooth out on top with a spatula. Sprinkle the remaining crushed mini eggs on top.
Bake for 20-25 minutes for an 8x8in pan, or 15-18 minutes for a 7x11in pan.** Be sure to check them early to make sure they don't overbake!
Allow the brownies to cool completely to room temperature before cutting them into bars.