Heat the oven to 180C/160C fan/gas 4 and line a large baking sheet with a silicone baking sheet. Write fortunes on pieces of white paper measuring 6cm long by 1cm wide.
Using an electric whisk, whisk the egg whites, vanilla and oil with ½ tbsp cold water until frothy, about 30 seconds. Put the flour, cornflour, sugar and a good pinch of salt in a bowl, then tip into a sieve and sift into the egg white mixture. Whisk to a smooth batter and chill for 1 hr.
Put ½ tbsp of mixture onto the prepared baking sheet. Using a small palette knife, push the mixture out into a thin 8cm circle. Repeat with another ½ tbsp of the mixture to make two cookies, spacing them well apart as they will spread. Bake two cookies at a time, as you will need to shape them while they are hot. Bake for 8 mins until the edges turn golden.
Using a palette knife, lift the cookies one at a time onto a board, flipping them over. Put a fortune in the middle of the circle, then fold the circle in half, pinching the two edges together to seal. Lift the cookie onto the rim of a mug or pan and very gently pull the two corners down to shape. Hold for 10 seconds, then transfer the cookies to muffin tins so that they hold their shape while cooling and setting. Repeat with the remaining cookie mixture.
Melt the white chocolate in the microwave or in a bowl set over a pan of simmering water. Tip the sprinkles into small, deep bowls. Dip the two ends of each cookie into the chocolate, shake off any excess, then dip into the sprinkles. Leave to dry on a board. Will keep for three days in an airtight container.