Preheat the oven to 350°F. Line two baking sheets with parchment.
In a large bowl or the bowl of a stand mixer, beat together the butter, brown sugar, and white sugar. Add the egg and vanilla and mix to combine.
Mix in the flour, cornstarch, baking soda, and salt until barely combined (there should be some flour streaks still). This should be a thicker dough. Add in an extra Tablespoon of flour if needed. Mix in the mini cadbury eggs and mini chocolate chips, just until combined.
Use a medium cookie scoop to scoop the dough into 1-inch balls (golf ball size about 1.5 ounces). For a bigger cookie you could do 2-3 ounce balls. Place on prepared baking sheets. Bake 8 to 10 minutes, or until golden around the edges.
Remove from oven and let cool on baking sheet before removing to a cooling rack. Store in a covered container up to 3 days.
Notes
Butter: Make sure to leave the butter out on counter hour before making or follow these tips on how to soften butter. If using salted butter, only use 1/2 teaspoon salt.
Cornstarch: The cornstarch in this dough helps the cookies not to spread, while also making the center extra soft.
Make ahead: This dough can be made 24 hours in advance and stored in the refrigerator until ready to bake.
Storage: Store leftover cookie sin an airtight container for 3 days or freeze up to 3 months. Thaw overnight in the refrigerator before enjoying, or leave at room temperature.
Freezing cookie dough: Cookie dough balls can also be stored in the freezer for 2-3 months. I like to place the dough on the baking sheet and freeze for 30-60 minutes, or until firm. Then you can stack in a freezer bag or covered container. When you are ready to bake, place on the pan while the oven is preheating and bake an extra 1-2 minutes.