Melt the butter in a saucepan over a medium heat, then leave it to cool for a few minutes.
Add the brown sugar and granulated sugar to a large mixing bowl. Pour in the melted butter and whisk until well combined.
Add the vanilla and egg, then whisk again until smooth.
Add the flour, baking powder, and bicarbonate of soda. Mix with a wooden spoon until a cookie dough forms.
Fold through the Rolos, then divide the dough into 10 pieces and roll each one into a ball.
Pop the cookie dough balls into the fridge for 30 minutes. Meanwhile, preheat the oven to 170°C.
Arrange the chilled cookie dough balls onto lined baking trays, leaving plenty of space between them. Don’t flatten them, keeping them as balls will help give you those thick, chunky centres.
Bake for 15 minutes, until golden around the edges but still slightly soft in the middle.
Leave them on the baking trays for at least 10 minutes before transferring to a cooling rack. They’ll firm up as they cool.