Preheat oven to 375°F. Line baking sheets with parchment paper or silicone baking mats. Set aside.
Add the butter, granulated sugar and brown sugar to a large bowl. Beat together, about 3 minutes on medium.
Add the egg and vanilla. Mix until well-combined, about 1-2 minutes on medium, scraping down the sides of the bowl as needed.
Add in the flour, baking soda, baking powder and salt
Fold in the chocolate chips and Cadbury eggs, and stir in until just combined.
Scoop out 2 tablespoon portions of dough.
Roll into a ball, and place the dough balls 3 inches apart on the prepared baking sheets.
Bake for 8-12 minutes, or until the cookies edges of the cookies are turning golden brown. The centers may look soft and underdone. That's ok.
Let the cookies to cool on the baking sheet for 10 minutes before removing to a wire cooling rack.
Notes
Butter: I use salted butter. The butter should be soft enough to dent when pressed, but the butter shouldn't be warm or losing its shape.
Brown sugar: I use light brown sugar, but dark brown sugar will also work.
Vanilla extract: Feel free to use less if you prefer. I wouldn't recommend using less than 1 teaspoon.
Flour: Properly measuring flour can make or break a recipe. The most accurate way to measure flour is to weigh it (click "metric" underneath the ingredients for the ingredient weights). To measure by volume, sift or stir the flour to break it up. Lightly spoon it into the measuring cup and level. You want to avoid packing in the flour. Too much flour will give you a dry, crumbly cookie dough and heavy cookies that don't spread.
Chocolate chips: You can use another type of chip, like white chocolate chips, instead.