Preheat your oven to 180°C and lightly grease your muffin tins.
In a large bowl, mix the melted butter and sugar and stir thoroughly until combined. Add the egg and vanilla, ensuring it’s mixed well. Slowly add the flour, chocolate chips and a pinch of salt and beat vigorously until a soft dough is formed.
Add one tablespoon of dough to each well of the muffin tin. Pack them down with the spoon and add two fudge pieces and a small pinch of sea salt to each well.
Add one more tablespoon of the cookie dough and mould over the fudge.
Bake in the middle of the oven for 10 minutes. The top should be crisp and golden but the inside will be oozy and gooey.
They are best served warm to keep the caramel soft, but you can warm them up in the microwave for 30 seconds if you need to.