Brown the butter: In a large pot, melt the butter over medium heat. The butter will start to foam and froth; as the foam dies down, you’ll see brown specks forming at the bottom and smell a nutty aroma. That’s when it’s ready.
Place the butter in a heat-safe bowl and let it cool. Make sure to use a rubber spatula to scrape out all those brown specks at the bottom, because that's where the flavor is.
Place your Cadbury mini eggs in a plastic zip top bag and crush them a little with a rolling pin. I like a good mix of big chunks and small chunks and then I save some of the really big chunks for sprinkling on top at the end.
Preheat the oven to 350°F. Line an 8x8 or 9x9 pan with parchment paper
Mix the slightly cooled browned butter with the brown sugar and granulated sugar until smooth and glossy (I found it easiest to use a rubber spatula for mixing, but a whisk or electric mixer work too).
Whisk in the eggs, one at a time, then stir in the vanilla.
Add the flour, baking soda, salt, and cornstarch (if using). Stir just until no dry streaks remain.
Fold in the crushed Cadbury Mini Eggs and chocolate chips. Keep some Cadbury Mini Eggs aside for sprinkling on top later.
Press the dough into your lined pan.
Bake for 18-20 minutes, until the edges look set and the center still looks soft.
Sprinkle the extra chopped Mini Eggs on top and return to the oven.
Bake for another 6–10 minutes, until the edges are golden and the center is still slightly underbaked