Beat softened butter on high speed in the bowl of your stand mixer. Add sugar, baking powder, and salt, when the butter has been whipped. Add egg, milk, and vanilla.

Slowly, add half a cup of flour until it's completely incorporated. Divide dough into two bowls, and add food colouring to one bowl. Mix well and refrigerate both bowls of dough for an hour.

With a rolling pin, roll out both chunks of dough. Using a knife, cut into identical rectangles. Brush a bit of water onto the coloured chunk of dough and place the uncoloured dough on top of it. Roll tightly and wrap with cling wrap. Refrigerate for an hour.

Preheat oven to 375 degrees F, and start cutting thin slices of the rolled up dough. Place cookies on parchment-lined baking sheets and bake for 7 to 8 mins.

Let cool completely before enjoying. Store in an airtight container.