Step 1 of 4

Bake the base first

Heat the oven to 350°F and line a 20 by 30cm pan. Rub the cold butter into the flour and superfine sugar until it looks like breadcrumbs, squeeze it into a dough, press it firmly and evenly into the pan and bake for 25 minutes until pale gold. Leave it to cool.

— An unbaked base under hot caramel does not cook, it steams. You cannot rescue it later, and the whole tray goes damp from the bottom up.