Preheat oven to fan assisted, 160C / 180C / 350F gas 4 and lightly oil your skillet.
In a large bowl, mix together the flour, sodium bicarbonate and salt and set aside.
Using an electric mixer cream the butter and sugar for a couple of mins until nice and smooth. Then add in the eggs and beat for a further min, before adding in the vanilla and peanut butter and continuing mixing till everything is well combined.
Tip the dry ingredients to the wet ingredients and continue mixing with the hand mixer until combined before tipping in the smarties.
Stop the hand mixer and transfer the cookie dough to the skillet, making sure to spread the dough evenly through the skillet.
Bake in the hot oven for approx 35 mins until lightly browned on top and the sides.
Let the cookie cool for a few mins in the skillet before serving with some cream, fruits and leftover smarties.
Very slighty adapted from Sally's Baking Addiction.