Put the margarine and the white sugar in a
saucepan and heat gently, stirring all of the
time. When all of the margarine has melted, add
the golden syrup and black treacle. Continue to
stir until the sugar has dissolved. Boil slowly,
over a low heat, stirring occasionally.
You need to boil off enough water for the
mixture to solidify when cooled to room
temperature but you also need to keep the
temperature of the mixture low enough during
cooking to prevent burning. The test involves
getting a sample of mixture on a teaspoon and
dropping it into cold water - either in a cup or
a saucer.
When the test yields a soft but not liquid
result, add the vinegar carefully. The water and
most of the acetic acid in the vinegar will boil
off but some will stay in the toffee and modify
the flavour - most importantly, the malt will
stay in the final toffee.
Lightly grease a toffee tray with margarine.
Keep on boiling slowly and testing every few
minutes (things speed up towards the end)
until you get a stiff result - you will get a
feel for how fast you can boil toffee without
burning it. Carefully pour into a greased toffee
tray and let cool. When cold, break up with a
toffee hammer or the handle of a heavy knife or
spoon.