Step 1

Using an electric mixer, beat butter, sugar and vanilla for 5 minutes or until light and fluffy. Add egg. Beat until just combined. Stir in flour, in 2 batches, until just combined. Divide dough in half. Using food colouring, tint 1 portion bright purple.

Step 2

Knead plain dough into a disc. Wrap in plastic wrap. Repeat with purple dough. Refrigerate for 30 minutes.

Step 3

Roll plain dough between 2 sheets of baking paper until 5mm thick. Remove top sheet of baking paper. Cut dough into a 30cm x 40cm rectangle. Set aside. Repeat with purple dough. Remove top sheet of baking paper. Turn purple dough, baking paper-side up, on top of plain dough. Remove baking paper. Using baking paper as a guide, roll up dough from 1 short end to form a log. Place sprinkles on a sheet of baking paper. Brush log lightly all over with egg white. Roll in sprinkles to coat. Wrap in baking paper. Freeze for 30 minutes or until firm (see note).

Step 4

Preheat oven to 180C/160C fan-forced. Line 3 large baking trays with baking paper. 5 Unwrap cookie log. Using a flat-bladed knife, cut into 1cm-thick slices. Place, 3cm apart, on trays. Bake, 1 tray at a time, for 10 to 12 minutes or until edges are just starting to turn light golden. Stand on trays for 5 minutes. Transfer to a baking paperlined wire rack to cool completely. Serve.