Begin by preheating your oven to 350°F (175C/155C Fan) with a cookie sheet inside.
In a large mixing bowl, cream together the softened butter and the brown sugar for about 2 minutes using a handheld electric beater (see note).
Add the egg and vanilla, beating to combine.
Sift the flour, cocoa powder, baking powder, baking soda and salt straight into the sugar mixture. Use a spatula to stir to form a thick cookie dough taking care to ensure you don't leave any patches of unmixed flour.
Add half of your crushed Mini Eggs to the dough and stir to distribute, reserving the remaining Mini Egg pieces for topping your cookies.
Divide your dough into five large, roughly equal balls. (see note)
Dampen your hands with some cool water. Taking one large cookie dough ball at a time, pick up the dough ball and press it into a disc. Hold this in the palm of your hand and place an unwrapped Cadbury Creme egg in the centre. Envelop the Creme Egg with the dough and roll with your hands to seal the cookie dough around the Creme Egg. Repeat with each of the dough balls until you have 5 Creme Egg stuffed cookie dough balls.
Dip the top of each of these in the reserved Mini Egg pieces to get a bit of a coating on top of the cookie dough.
Place a piece of parchment paper on your preheated cookie sheet and carefully place your cookie dough balls well spaced on this (at least 3" apart).
Bake in the centre of your preheated oven for about 13 minutes.
Remove from the oven and allow to cool on the pan for at least 5 minutes before moving to a wire rack to finish cooling. (see note)