Cream butter and sugars: In a large bowl, beat the butter, brown sugar, and granulated sugar until smooth and fluffy, about 2–3 minutes.
Add eggs and vanilla: Mix in the eggs and vanilla until just combined.
Add dry ingredients: Add the flour, cornstarch, baking soda, baking powder, and salt. Mix until a soft dough forms.
Fold in mix-ins: Stir in the chopped Cadbury Mini Eggs and chocolate chips.
Scoop and chill: Scoop large portions of dough (about 1/4 to 1/3 cup each) onto a parchment-lined baking sheet. If time allows, chill for at least 30 minutes.
Preheat oven: Preheat oven to 375°F and line baking sheets with parchment paper.
Bake: Bake for 10–11 minutes, until the edges are set and centers are slightly soft.
Cool and finish: Let cool on the pan for a few minutes, then press extra Mini Egg pieces on top and sprinkle with sea salt, if desired.