Pre-heat the oven to 160 degrees C. Grease and line your 6" round cake tin.
Place the flour, sugar, bicarbonate of soda into a bowl & stir to combine.
In a jug, whisk together the milk, vanilla extract, melted margarine and vinegar.
Pour the wet ingredients into the bowl with the dry ingredients and mix together until smooth.
Place half of the mixture back into the jug & mix the cacao powder into one half of the mixture, so you have one vanilla batter and one chocolate batter.
Place one spoonful of vanilla batter into the tin, then one spoonful of chocolate batter. Repeat until all the batter is in the cake tin. Use a skewer or end of a spoon to swirl the batters together to create a marble effect.
Bake in the oven for 45-60 minutes until the top is golden brown.
Leave to cool in the tin, then turn out onto a plate or serving dish.
You can decorate the cake to match the current phase of the moon. For a full moon, dust the whole top with your icing sugar. For a crescent moon use a plate or saucer & cover the majority of the top of the cake before dusting.
Notes
You can use a 6", 7" or 8" round tin. The larger the tin, the thinner the cake will be, so reduce your cooking time to suit.