1. Preheat oven to 180°C/350°F/Gas 4. Lightly grease a 25cms x 20cms/ 10" x 8" roasting tin or deep cake tin.
2. Sieve the flour and baking powder into a bowl and leave aside.
3. In a large mixing bowl, beat together the sugar and margarine until creamy, scraping down the bowl occasionally!
4. Add the eggs two at a time, beating well after each addition. Stir in the vanilla essence.
5. Add the flour mixture alternately with the milk, stirring well after each addition.
6. Pour mixture into the prepared tin and bake for about 45 to 55 minutes or until a knife or toothpick inserted into the centre comes out clean!
7. Cool completely.
8. Meanwhile make the topping by melting the margarine and chocolate together over a low heat. Gradually beat in the icing sugar alternately with the milk and vanilla essence. Allow topping to get cold and thicken.
9. Spread over cold sponge and cut into squares.

Notes:

As an alternative, these squares can also be made without the topping and some sliced strawberries and cream placed on top of each square and served for dessert.