Preheat the oven to 180ºC. Lightly grease a 20cm x 30cm shallow tin and line the base and two sides with baking paper.
Combine the base ingredients in a food processor, or rub the butter into the flour and icing sugar and work into a smooth paste with your fingers. It should be fairly soft and malleable. Press the dough evenly into the prepared tin.
Bake for 15–20 minutes until lightly golden. Don’t overcook as it is going back into the oven later.
Combine the topping ingredients in a bowl, mixing with a fork until well amalgamated. (If you make the topping in advance, make sure to mix it again before you pour it on or all the sugar will stay at the bottom of the bowl.)
Pour the mixture onto the hot biscuit base and return to the oven for another 30 minutes.
Remove from the oven and place the tin on a wire rack to cool. Dust the top with icing sugar and chill for at least 1 hour then cut into bars.