In a small saucepan set over medium heat, brown the butter. Stir the butter occasionally until it turns amber in color and develops little brown specks.
Transfer the brown butter to a medium sized mixing bowl and set it in the freezer for 5 minutes to chill. You should be left with at least 90g of brown butter, if you have a little less (2-3g), just add in some water until you reach 90g. If you have way less than 90g, you cooked the butter for too long and will need to start again.
Once the butter has cooled down a bit, whisk in both sugars.
Whisk in the egg and vanilla until well combined.
Add in the flour, baking soda, baking powder and salt. Use a rubber spatula to fold the mixture together just until a few flour streaks remain.
Add in the crushed cadbury eggs and chopped chocolate and fold just until no flour streaks remain.
Cover the bowl tightly with plastic wrap and place it in the fridge for 30 minutes to firm up.*
Once you're ready to make the cookies, preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper.
Scoop out heaping 3 tbsp. scoops of dough**, setting them on the cookie sheet at least 2" apart to allow room for spreading. (If you chilled the dough for longer, you may need to wait 10-20 minutes for it to become soft enough to scoop.)
Top each cookie with additional crushed cadbury eggs if desired, and bake for 10-12 minutes, or until the edges of the cookies start to turn golden brown.
Once the cookies come out of the oven, I recommend "scooting" them. This just means take a bowl, cup, or biscuit cutter that is slightly larger than the cookie, place it on top of the cookie, and swirl the bowl around to make the cookie a little more round.
Press a chocolate bunny into the cookies while they are still warm, about 5 minutes after taking them out of the oven. Serve and enjoy!