For the cake:
Unsalted Butter (softened)
Caster sugar (although it doesn't go badly with granulated sugar if that's all you've got)
You should use the same weight for all the ingredients. Because all eggs are different sizes, I find the best way to work out the quantities is to weigh the eggs. and then use the weight of the eggs to determine how much of everything else to use.
For the buttercream:
200g Icing Sugar
100g Unsalted Butter (softened)
1) Preheat your oven to gas mark 5, 190c, 170 fan.
2) Grease two 20cm round tins and line the bottom with non-stick greaseproof paper.
3) Put the butter, sugar, eggs and flour in a mixing bowl and whisk with a hand mixer for a couple of minutes until everything is combined and there are no lumps of butter left in the mixture.
4) Divide the mixture evenly between the two tins and smooth with a spatula. Bake in the oven for 20-25 minutes until a cocktail stick/skewer comes out clean or the top springs back when lightly pressed.
5) Turn out on a wire rack and leave to cool. To make the buttercream, put the butter and icing sugar in a bowl and mix with a fork or hand mixer until smooth and no lumps are left. Spread jam over the bottom of one cake and buttercream over the bottom of the other, then sandwich the two sponges together. Dust with a little icing sugar.