Put the oven on 180C/350F/Gas Mark 4.
MAKE THE SHORTBREAD BASE
Cream butter and sugar until pale and fluffy. Add flours and salt, a little at a time, until fully mixed, smooth and uniform.
Press intothe bottom of a greased and lined 8in square cake tin. Place in fridge for 30 mins to firm it up.
Bake for 30-35 mins until pale golden brown. Remove from oven and leave to cool in tin.
MAKE THE TOFFEE FILLING
Gently heat the butter, sugar and condensed milk together until sugar has dissolved. bring to the boil, and simmer for 5 mins, stirring continuously. Leave to cool for one minute, then pour over the shortbread. Leave to cool.
MAKE THE CHOCOLATE TOPPING
Place the chocolates in different bowls and melt (I use the microwave and frequently check and stir).
Spoon each chocolate over the toffee layer, tilting the tin so chocolate covers the whole layer. Using a knife, swirl the chocolate for a marbled effect. Leave until chocolate is just set, and, using a sharp knife, mark into squares. Leave to set for around an hour, then cut into squares.
Rice flour improves the texture of the shortbread. You can leave this ingredient out and increase the amount of plain flour instead.
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