Line a couple of baking trays and whip up your butter and sugar until really light and fluffy- probably around 5 minutes with an electric mixer.
Beat in the egg and vanilla paste until fully incorporated. Sieve in the flour and bicarb and slowly mix through. Chuck in the choc and do the same adding the 1 teaspoon salt in too.
Grab tablespoon. of the dough and roll into balls. Smush in a rolo into the middle of each one and add a sprinkle of sea salt on the top. Leave the chill in the fridge until hard.
Preheat the oven to 190c and bake for around 10-12 minutes for perfectly crunchy edges and the gooey middles.