In a food processor, whizz the butter,
icing sugar and vanilla until smooth.
Add 1 egg and blend to combine, then
tip in the flour and blitz once more.
Scoop the dough into a ball and wrap
in cling film. Chill for 30 mins.
Heat oven to 200C/180C fan/gas 6.
Dust 2 large baking trays with flour. Break
the dough into large cherry-sized pieces.
Roll into balls and put on the baking
trays, spaced a little apart. Squash down
the balls slightly with 3 fingers. Bake (in
batches, if necessary) for 8 mins – the
cookies will still be very pale at this stage.
Remove the cookies from the oven.
While still warm, quickly brush the tops
with the beaten egg, then dip half in the
hundreds & thousands, and press a few
Smarties onto each of the remaining
cookies – do this while the egg is still
wet, so the toppings will stick once
cooked. Bake for a further 4 mins.
Leave to cool on wire racks.
To make the icing, beat together the
butter, icing sugar and vanilla. Once the
cookies are completely cool, use the icing
to sandwich together the 2 different
cookies. Un-iced cookies can be made
up to 4 days ahead and kept in an
airtight container. You can fill with
the icing up to 1 day ahead.