Position the oven shelves towards the centre of the oven. Preheat the oven to 180°C/fan oven 160°C/Gas Mark 4. Lightly grease two 18cm (7in) sandwich tins with a little Trex, then line the bases with greaseproof paper.
Sift the flour, baking powder and salt into a mixing bowl. Add the sugar, Trex, unbeaten eggs, milk and vanilla extract. Use a wooden spoon to beat the mixture together for 1-2 minutes until smooth and creamy.
Divide the mixture between the prepared tins and level the tops. Bake for 18-20 minutes. To test that the cakes are cooked, they should be golden brown, and when touched lightly the tops should spring back into place.