Add the flour, baking powder, cinnamon and salt and gently mix until combined.
In a large bowl, rub together the butter, flour, cinnamon and vanilla until nice and crumbly, and then stir in the sugar.
For the icing:
Beat together the butter and brown sugar with the cinnamon and vanilla extract, for 5-7 minutes until fully combined and light and fluffy.
Add the icing sugar a little bit at a time and continue to beat on a low speed.