Heat oven to 180C/160C fan/gas 4. Line 2-3 baking sheets with baking parchment. Put the butter and sugar in a food processor and whizz until light and fluffy. Add the egg and vanilla, and mix well. Sift together the flour and custard powder, then tip into the bowl and pulse to mix into a dough. Scrape out the food processor and work the chocolate in by hand.
Roll the dough into balls a little smaller than a walnut, then place on the baking sheets, a little apart to allow for spreading. Press each biscuit down lightly with your fingers.
Bake for 12-15 mins until lightly golden. Remove and cool on a wire rack.