Preheat your oven to 350°F. Grease and line a 9x13 metal pan with parchment paper.
In a stand mixer, combine the melted butter, brown and granulated sugar. Mix until it's all combined.
Add in vanilla and eggs, and mix until everything is nicely incorporated.
While the mixer is on low, add in the flour, salt, cornstarch, and baking soda. Mix until just combined.
Fold in the chocolate chips and Cadbury Eggs.
Scoop the batter into the prepared pan and use a spatula to ensure it's evenly spread out.
Bake for 22-25 minutes or until the edges are golden brown. Let it cool completely before frosting.