Lightly grease and base line a 20 cm/8 in sandwich tin.
Cream 175 g/6 oz butter or margarine with the sugar until very soft, light and fluffy.
Sift together the flour and rice flour, then gradually work these dry ingredients into the creamed mixture to make a soft dough.
Press the dough into the prepared tin and smooth the surface with the back of a warm metal spoon.
Chill for 1 to 2 hours.
Meanwhile, melt the remaining fat, together with the marmalade and syrup, in a saucepan over a low heat.
Allow to cool slightly, then stir in the cornflakes.
Spread this mixture evenly over the surface of the shortbread.
Bake in a moderate oven (170 C, 325 F, gas 3) for 50 minutes, or until darkened in colour and cooked.
Mark and cut the hot marmalade crunch into 8 portions and leave to cool completely before removing from the tin with a palette knife.