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.
Let the butter sit until it is warm, not hot, at least 30 minutes. This keeps the cookies from spreading too much in the oven.
Place your Cadbury mini eggs in a plastic zip top bag and crush them with a rolling pin.
Preheat the oven to 350°F. Grease a 10-inch oven-safe skillet.
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.
Press the dough into the greased skillet in an even layer and smooth the top.
Bake for 14–16 minutes, until the edges look set and the center still looks soft.
Pull your skillet out of the oven briefly. Sprinkle the extra chopped Mini Eggs on top and gently press them in so they stay in place, but not much so they sink into the dough.
Stick the skillet back in the oven and bake for another 6–10 minutes, until the edges are golden and the center is still slightly underbaked.
Cool 15–20 minutes, then slice into wedges or serve warm with spoons and ice cream.