Preheat the oven to 180C/350F/gas mark 4. Grease two 20cm round tins and line the bases with baking parchment.
Weigh the eggs in their shells then measure out the same weight of each butter, caster sugar and self-raising flour.
Place the butter and sugar in a large bowl and whisk with an electric mixer until very pale and fluffy.
Whisk in the eggs one at a time, adding a spoonful of the flour with each one and mixing well after each addition. Whisk in the vanilla extract (if using)
Sift together the remaining flour, the baking powder and salt. Add it to the bowl and fold in gently by hand.
Add enough milk so that the mixture drops slowly off a spoon but doesn't run off.
Divide the batter evenly between the tins and spread it level. Bake for about 25 minutes until golden and well risen, a skewer inserted into the center should come out clean.
Leave to cool in the tins for 10 minutes then carefully turn out onto a wire rack and leave to cool completely.
To make the buttercream, mix together the butter, icing sugar, vanilla extract and 1 tbsp milk on a low speed until combined then turn up the mixer to a high speed and whisk for a good few minutes until extremely light, pale and fluffy. Add a drop more milk if it seems too stiff.
Reserve the better looking cake layer and level the top of the other if needed. Place it on a serving platter or cake stand and spread with a generous layer of jam.
Transfer the buttercream to a disposable piping bag and snip off the tip (or place in a bag fitted with a large, plain nozzle). Pipe dollops of the buttercream on top of the jam then gently place the other cake layer on top. Sprinkle with a little caster sugar then serve.