Preheat the oven to 160°C and cover a baking sheet with baking paper.
1. Cookies: Cream together the butter and sugar and beat until light and fluffy. Add the egg and vanilla essence and mix until well combined.
2. Sift together the flour, salt and baking powder. Add this mixture one cup at a time to the wet mixture until the dry ingredients are completely incorporated. Add the sprinkles and mix until just combined. Wrap in cling film and chill in the fridge for an hour.