Heat oven to 200C/180C fan/gas 6. Mix
the butter, sugars, egg yolks and vanilla
with a wooden spoon until creamy, then
mix in the flour in 2 batches. Roll out
thinly on a floured surface, then use a
standard 30cm ruler as a template to
cut the dough into small, even squares.
Do this by starting with the ruler flush
with one side and cutting along the
length of it. Repeat across the width of
the dough, then do the same from the
top down. Transfer to baking sheets and
bake for 8-10 mins until golden.
While the biscuits cool, mix the butter,
icing sugar, custard powder and food
colouring, if you have any. Pipe or spread
a little icing onto a biscuit, then sandwich
with 1 or 2 more biscuits. Repeat until
all the biscuits are used, then dust with
a little more icing sugar.