Preheat oven to 350 degrees. Line a cookie sheet with parchment paper or a silicone baking mat. In a large bowl (using a hand mixer) or in a stand mixer, cream the butter and sugar. Beat in egg (preferably at room temperature), then add the vanilla extract. Add the rest of the dry ingredients and mix. The dough will seem crumbly at first, but keep mixing–it will come together to be a thick dough.
Smash the Cadbury mini eggs with a rolling pin until they’re in medium-sized chunks (put them inside a large plastic bag), then pour them into the cookie dough and stir well. Use a small or medium cookie scoop to evenly place them on the baking sheet. Add a few more egg pieces on top of the cookies and press in slightly. It makes for a prettier cookie, and gives you even extra chocolate!
Bake cookies for 10-12 minutes at 350 degrees. Remove from baking sheet after a few minutes, and cool on a cooling rack. Store in an airtight container.