Preheat the oven to 160°C fan / 180°C conventional. Line a 20x24cm (or similar size) baking tin with greaseproof paper.
Using an electric whisk, beat the margarine (or butter) and caster sugar together until light, fluffy and creamy.
250 grams Margarine or unsalted butter, 250 grams Caster sugar
Add the eggs, flour and vanilla. Beat until smooth and combined.
250 grams Self-raising flour, 4 medium Eggs, 1 teaspoon Nielsen-Massy Vanilla Extract
Spread the mixture into the baking tin and bake for 35-45 minutes until risen, golden on top and the middle of the sponge springs back when lightly pressed. If you know your oven runs hot, bake until the 35-minute mark. If your oven runs cool, bake for up to 45 minutes. Not sure? 40 minutes is a safe bet. Leave the cake to cool completely in the baking tin.