Step 1: Prepare the Cake Tins

• Preheat the oven: Set your oven to 180°C (160°C fan)/350°F/gas mark 4.
• Prepare the tins: Grease and line two 20cm round cake tins with baking parchment.

Step 2: Whisk the Eggs and Sugar

• Set up a bain-marie: Place a heatproof bowl over a pan of simmering water (making sure the bowl doesn’t touch the water).
• Whisk the eggs and sugar: Crack the eggs into the bowl and add the caster sugar. Using an electric whisk, beat the eggs and sugar continuously over the heat for about 5-7 minutes, until the mixture is pale, thick, and has tripled in volume.

Step 3: Fold in the Dry Ingredients

• Remove from heat: Once the egg mixture is thick and holds its shape, remove it from the heat.
• Fold in the flour: Sift the plain flour and salt over the egg mixture and gently fold it in using a spatula. Be careful not to deflate the batter.
• Add the melted butter: Drizzle the cooled melted butter and vanilla extract (if using) into the batter, gently folding until fully incorporated.

Step 4: Bake the Sponge

• Divide the batter: Evenly divide the batter between the two prepared tins and smooth the tops with a spatula.
• Bake: Place the tins in the preheated oven and bake for 20-25 minutes, or until the cakes are golden and springy to the touch.
• Cool the cakes: Allow the cakes to cool in the tins for 10 minutes before turning them out onto a wire rack to cool completely.

Step 5: Fill and Assemble (Optional)

• Layer with your favorite fillings: Once the cakes are completely cooled, you can layer them with whipped cream, fruit, or buttercream.
• Dust with icing sugar: For a simple finish, dust the top of the cake with icing sugar before serving.