Pre-heat the oven to 180°C. Grease the tray with margarine and some flour/cocoa powder. Beat the margarine and sugar well. Add one egg at a time. Beat well after adding each egg. (Don't put all eggs together). Add vanilla. Sift the flour, cocoa powder and baking powder together. Stir all together to the mixture. Put the milk, baking soda and cream of tartar in a small bowl and heat for about 15 seconds in the microwave until it starts to come to the top of the bowl. (Have a look while its heating because it might rise up and spill over in less than 15 seconds, in which case you have to stop the microwave before 15 seconds). Mix this with the cake mixture. Place the mixture in the tray and bake for about 30 minutes. Place a skewer or toothpick through the middle of the cake. If it comes out wet, its not fully cooked (put it back to the oven). If the skewer is clean with only a few crumbs coming out, its done (take it out of the oven). Cool for at least 3 hours (probably more) before putting icing.
Chocolate icing 250g margarine 500g icing sugar 50-100g cocoa powder or more (depending on desired colour) 1 teaspoon vanilla 1-2 tablespoons milk Beat the margarine and icing sugar till light and fluffy. Add the sifted cocoa powder (first add 50g and then add more if needed) and beat well, then add vanilla and milk and beat again.