In advance, make the brown butter. Here's how: Take 1/2 cup of butter (1 stick) and place it into a small saucepan. Over medium heat, melt the butter, about 2 minutes. It will begin to bubble, then it will start to foam. When the foam appears, stay close and gently stir the butter until it starts to turn a dark amber color and smell slightly nutty. Immediately remove it from the heat and add the browned butter to a dish to cool back into a solid (I just pop mine into the freezer for 15 minutes or so). Make sure to scrape up any dark bits from the bottom - that's where all the flavor comes from!
Prep two large aluminum cookie sheets with parchment paper and set aside.
Preheat oven to 410 degrees (yep, that's right!)
In a stand mixer fitted with a paddle attachment, whip up the butter for 1 minute until it's light and fluffy, then add in the sugars to cream together with the butter for 3 minutes on high speed.
Scrape down the sides of the bowl and then add in the vanilla and eggs. Mix again until combined.
Add in the flour, cornstarch, baking soda, and salt and slowly mix together on the very lowest mixer speed until just combined (don't overmix!)
Reserve 1/8 cup of mini chocolate chips, chocolate chips, and a handful of mini eggs on the side, then mix in the rest on the lowest speed. Split the dough into 8 balls (baseball-sized) and place 4 on each cookie sheet. Hand-press on the chocolate chips and Cadbury eggs.
Bake for 7-9 minutes. They should be slightly golden on the outside, slightly mounded in the middle, and thick/tall, and gooey in the center. Let them cool on the pan for 1 minute, then carefully transfer the cookies to a cooling rack to finish cooling. Enjoy!