In a medium bowl, mix the flour, cocoa powder, baking soda, baking powder, cornstarch, and espresso powder. Whisk to combine and set aside.
In the bowl of a stand mixer equipped with a paddle attachment, or using a hand mixer, cream the butter, granulated sugar, and brown sugar on medium high speed until light and fluffy. About 2-3 minutes.
Add the vanilla and eggs and beat until just combined.
Add the dry ingredients in two parts, mixing just until there are no dry spots remaining. Scrape the sides and bottom of the bowl and mix until all ingredients are incorporated.
Add the mini chocolate chips and Cadbury eggs last and mix until they are evenly distributed throughout the dough.
Cover the dough with plastic wrap and refrigerate for at least one hour to overnight.
Once the dough is chilled, preheat your oven to 350 degrees F. and prepare a large baking sheet by spraying with non-stick spray.
Use a large cookie scoop (I used a 3 tablespoon scoop) and scoop out even balls of dough and place them about 3 inches apart on the baking sheet.
Bake for 10-12 minutes or until the edges are golden brown. Remove from the oven and place on a cooling rack for 15 minutes.
Transfer the cookies to a cooling rack after 15 minutes and allow to cool completely.
Keep stored in an air tight container at room temperature for up to three days.