Prep: Preheat oven to 350°F and line a baking sheet with parchment paper. Set aside.
Dry mixture: In a small bowl, stir together flour, cornstarch, baking soda, and salt. Set aside.
Cream butter and sugar: In a large mixing bowl, use an electric hand mixer to beat the butter until it becomes very creamy and pale in colour. Add the granulated sugar and brown sugar and beat on medium speed until creamy and fluffy, about 3 minutes.
Eggs: Add the whole egg, egg yolk, and vanilla extract and mix on low speed until combined.
Combine dry and wet: Add the flour mixture and briefly mix on low speed until almost combined but with some patches of flour remaining.
Chocolate: Add the crushed mini eggs and chocolate chunks. Switch to a spatula to finish folding the dough together until just combined.
Scoop cookie dough: Use a 4-tablespoon cookie scooper to scoop out balls of cookie dough. Release the dough balls on your lined baked sheet, leaving about 2” in between each ball to account for spreading. Press a few pieces of chopped mini eggs on top of each dough ball.
Bake: Bake cookies for about 12 minutes or until the edges are set and lightly golden brown but the middle is still very soft. In the last 2 minutes of baking, press a few extra pieces of chocolate on top of each cookie (optional).
After baking: Immediately after taking them out of the oven, use a large round cookie cutter to swirl the cookies to make them perfectly round or use a fork to push in any uneven edges while the cookies are still soft. Press a few extra mini eggs on top of each cookie and sprinkle any remaining pastel mini egg "dust" on top of the cookies for a picture-perfect look.
Cool: Let cookies cool for 5-10 minutes before transferring them to a wire rack to cool completely or enjoy while still warm.