Mix butter and sugar until creamy. Beat in egg. Add the rest of the dry ingredients and mix. It will seem crumbly at first, but keep mixing--it will come together to be a thick dough.
Smash the mini Cadbury eggs with a rolling pin until they're in medium-sized chunks (put them inside a large plastic bag), then pour them into the dough and combine well.
Bake 10-12 minutes at 350 degrees. Cool on a wire rack.