Heat oven to 200C/180C fan/gas 6.
Mix the butter, sugars, egg yolks and
vanilla with a wooden spoon until
creamy, then mix in the flour in
2 batches. Stir in the orange zest.
Roll the dough into about 22 walnut-size
balls and sit on baking sheets. Bake for
15 mins until golden, then leave to cool.
Meanwhile, mix the icing sugar with
enough orange juice to make a thick,
runny icing. Dip each biscuit half into
the icing, then straight into the sprinkles.
Dry on a wire rack.