Preheat the oven to 180C/160C Fan/Gas 4. Grease and line two 20cm/8in sandwich tins. Use a piece of baking paper to rub a little butter around the inside of the tins until the sides and base are lightly coated, then line the bottom with a circle of baking paper.
Break the eggs into a large mixing bowl, then add the sugar, flour, baking powder and butter. Mix together until well combined with an electric hand mixer (you can also use a wooden spoon), but be careful not to over mix. Put a damp cloth under your bowl when you’re mixing to stop it moving around. The finished mixture should fall off a spoon easily.
Divide the mixture evenly between the tins: this doesn’t need to be exact, but you can weigh the filled tins if you want to check. Use a spatula to remove all of the mixture from the bowl and gently smooth the surface of the cakes.
Bake the cakes on the middle shelf of the oven for 25 minutes. Check them after 20 minutes. The cakes are done when they’re golden-brown and coming away from the edge of the tins. Press them gently to check – they should be springy to the touch. Set aside to cool in their tins for 5 minutes. Run a palette or rounded butter knife around the inside edge of the tins and carefully turn the cakes out onto a cooling rack.
To take your cakes out of the tins without leaving a wire rack mark on the top, put the clean tea towel over the tin, put your hand onto the tea towel and turn the tin upside-down. The cakes should come out onto your hand and the tea towel – then you can turn them from your hand onto the wire rack. Set aside to cool completely.
To assemble the cake, place one cake upside down onto a plate and spread it with plenty of jam. If you want to, you can spread over whipped cream too. Top with the second cake, top-side up. Sprinkle over the caster sugar.
To freeze the cakes: Allow the cakes to cool completely, then wrap each cake well in clingfilm or freezer bags. Freeze for up to 6 months. Allow to defrost thoroughly before filling.