Preheat the oven to 160c/325f/Gas 3 and grease the tins.
Sift the flour into a mixing bowl and rub in the margarine.
Add the sugar and continue to mix until the ingredients come together as a stiff dough.
Roll out the mixture to fit the tins. Tip: this base can be hard to handle so if you have a small rolling pin finish rolling the base out once it's in the tin, or use the back of your hand to press to fit. Trying to get it even.
Bake in the preheated oven for 15 minutes then leave to cool. Make sure you only lightly brown the base.
For the topping, place the margarine, caster sugar and milk in a pan and melt over a low heat.
Pour the melted ingredients over the icing sugar and chocolate in a bowl; beat together for a minute. Spread the topping over the cooled base.
When cool and set, divide into portions and serve.