In a medium bowl, sift together the flour, baking powder and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter until smooth, about 2 minutes. Add the sugar and continue beating until the mixture is light and fluffy, about 3 minutes. Add the eggs one at a time, beating between each addition, then add the vanilla. Turn the mixer off. Add the flour and then beat just until combined. Remove the dough and separate it into two equal pieces. Add a few drops of colour to the one half. Roll first half out into a rectangle between two sheets of plastic.Roll the second coloured half out between another two sheets of plastic.Place coloured dough over plain dough.Starting from one end roll up like a swissroll. Refrigerate 30mins.Shake sprinkles onto a flat oven sheet. Remove dough from refridgerator and roll onto sprinkles making sure to coat evenly.Slice into 1cm thick slices and lay flat on oven tray.Bake at 180°C for 12-15 minutes till lightly browned. Allow to cool 10 minutes then remove to a cooling rack.