Step 1: In a large bowl or stand mixer, cream together the softened butter, granulated sugar, and brown sugar until the mixture is light and fluffy (about 3-5 minutes).
Step 2: Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.
Step 3: In a separate medium bowl, whisk together the flour, cornstarch, baking soda, and salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 5: Gently fold in 1 cup of the mini eggs by hand using a spatula.
Step 6: Scoop rounded tablespoons of dough (about 2 tablespoons per cookie) and place them on a tray. Cover and chill in the refrigerator for at least 2 hours.
Step 7: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Step 8: Place chilled dough balls on the prepared sheets, spaced 2 inches apart. Press the remaining 1/2 cup of mini eggs into the tops of the dough balls.
Step 9: Bake for 9-11 minutes, or until the edges are just golden brown but the centers still look slightly soft.
Step 10: Allow the cookies to cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely.