Preheat the oven to 370℉/180℃. Use oil to grease 8 holes of a muffin tin.
In a large bowl, or the bowl of a stand mixer, combine the butter, brown sugar and granulated sugar (medium speed) for about 3-5 minutes until fully combined. Add the egg and vanilla and mix again.
Fold in the flour, baking soda and cornflour. Then gently fold in the chocolate chips. Use an ice cream scoop or your hands to form 8 balls of dough. Place into the muffin tin.
Bake for 15-18 minutes, until golden on top.
Remove from oven, and use a small jar, tablespoon measure, end of a wooden spoon or similar to create indentations in the middle of each cookie cup.
Loosen the peanut butter by placing in a small bowl in the microwave for 30 seconds. Scoop a teaspoon into each cookie cup.
Melt the dark chocolate and spoon over each cookie cup to completely cover the peanut butter underneath. Let firm up in the fridge then serve!