Preheat your oven to 350 degrees Fahrenheit. Line an 8x8 or 9x9 inch baking pan with parchment paper, pressing it into the corners and up the sides.

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar for about 3 minutes until light and fluffy. You can use a stand mixer with paddle attachment, a hand mixer, or a sturdy spoon.

Add the egg and vanilla extract, mixing until well combined.

Gently fold in the flour, cornstarch, baking soda, and salt until just incorporated. Then fold in the milk chocolate chips and Easter mini eggs, being careful not to overmix.

Press the cookie dough evenly into the prepared pan. For thicker bars, use the 8x8 inch pan. If desired, press a few extra mini eggs onto the top of the dough before baking.

Bake for 18 to 22 minutes, until the edges are lightly golden and the center is still soft. The bars will continue to set as they cool; underbaking slightly ensures a chewy texture.

Let the bars cool completely in the pan on a wire rack, then cut into 16 squares using a plastic knife or a knife dipped in hot water and wiped dry for clean slices.