Add the sugar to the Thermomix bowl. Mill for 10 seconds / speed 9.
Add the butter, egg, vanilla extract, flour, baking powder and salt to the bow. Mix for 10 seconds / speed 7.
Scrape down the bowl and repeat for 5 seconds / speed 7.
Turn the dough out onto a floured surface and divide into two equal portions.
Return one portion of dough to the Thermomix bowl. Add the food colouring and mix for 5 seconds / speed 7. Then turn out onto the floured surface.
Use your hands to bring each portion of dough together and shape it into a rectangle.
Place one rectangle of dough onto a sheet of baking paper and use a rolling pin to roll it out into a large rectangle approximately 10" x 8" (25 x 20cm) and ¼ inch thick. Repeat with the other portion of dough.
Now carefully place one rolled out piece of dough onto the other and use your hands to gently press the two pieces together.
With the shorter edge facing you, carefully roll up the dough away from you to create a sausage shape.
Wrap it in plastic wrap and once covered, you can gently roll the pastry back and forth on the counter to ensure a nice uniform round edge all over. Place the dough in the fridge for approximately 2 hours.
When you are ready to bake the cookies, pre-heat the oven to 170 C / 350 F.
To coat the roll with sprinkles, pour out the sprinkles into a baking sheet and gently roll the cookie dough in, gently pressing down to encourage the sprinkles to sink into the dough a little.
Use a sharp knife to cut the cookie dough log into even slices around ¼ inch thick.
At this stage you can do a second roll of the edge of the cookie in the sprinkles, to ensure it is evenly coated.
Lay the cookies out on a baking sheet lined with baking paper a couple of inches apart and bake in the oven for around 10 minutes.
Allow the cookies to cool on the baking sheet for a couple of minutes and then transfer to a cooling rack to cool completely.