Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the egg and vanilla extract until fully incorporated.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the crushed Cadbury Mini Eggs and semi-sweet chocolate chips by hand.
Scoop rounded tablespoons of dough (approximately 1.5 ounces) and place them 2 inches apart on the prepared baking sheets.
Bake for 10-12 minutes or until the edges are lightly golden but the centers still look slightly soft.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.