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