Put the butter, vanilla and sugar in a large mixing bowl and stir with a wooden spoon until smooth. Add the ground rice, then stir in the flour and sprinkles. If the mixture starts to get dry, use your hands to bring everything together and make a smooth dough.
You can now choose between two different types of biscuit. If you want to make round ones, divide the dough in two. Lay out a piece of parchment or foil and shape the first lump of dough into a log, about 4-5cm across. Roll it up and chill for at least 30 mins. Repeat with the second piece of dough. If you want to make mini biscuits, roll out the dough on the parchment in a single layer about ½ cm thick. Wrap and chill as above.
Heat oven to 180C/160C fan/gas 4. Either slice the logs into rounds about ½ cm thick, or cut the slab into small squares, about 3cm each side. Bake on lined baking sheets for 20 mins for the large biscuits or 10 mins for small ones, until lightly golden. Leave to cool for 5 mins on the baking sheets, then slide onto racks to cool completely.