Preheat the oven to 180°C (fan oven) and lightly grease a 10inch cast iron skillet pan* with butter or spray oil.
Place the unsalted butter in a large microwave-safe bowl. Microwave for 30-60 seconds until the butter has melted.
Add the caster sugar and light brown sugar into the melted butter and mix until combined.
Mix in the whole egg, both egg yolks and vanilla extract.
Now fold in the flour and salt until the mixture comes together to form a cookie dough.
Transfer 200g of the Mini Eggs into a ziplock sandwich bag and bash with a rolling pin until roughly broken up. You'll want to keep the remaining 40g Mini Eggs whole to decorate the top of the cookie.
Fold the broken Mini Eggs into the cookie dough.
Tip the dough into the skillet pan and press it out using your hands.
Bake for 25 minutes until pale golden on top. The cookie should still be slightly wobbly, this will make sure it stays nice and gooey inside.
As soon as the cookie is out of the oven, gently press the remaining Mini Eggs into the top.
Leave to cool for 10 minutes, then top with ice cream and serve!