Preheat the oven to 160 C / Gas 3. Line 2 baking trays.
Melt butter in a saucepan over medium heat (or in the microwave). Set aside to cool slightly.
Pour butter into a large bowl and stir in brown sugar and caster sugar; stir until smooth and sugar has mostly dissolved.
Lightly beat egg with a whisk or fork; stir into the sugar mixture along with the vanilla.
Sift flours and baking powder together into a small bowl; gradually mix into sugar mixture until combined.

And I usually make the dough like sausage shape and cut after it and line on the train and after my boys add some chocolate /caramel chips
Bake for 12 minutes or until golden; remove from oven and allow to cool on trays for 3 minutes before transferring to a wire rack to cool completely.