Preheat the oven to 180 C / Gas 4. Line two sandwich tins with baking parchment and put to one side.
Cream together butter and sugar till light and fluffy.
In a separate bowl whisk the eggs then add the vanilla. Add to the butter mixture and whisk until smooth. Sieve the flour into the mixture and fold flour in until there is no dry flour left. Whisk until light and fluffy. Divide the mixture between the two prepared tins.
Place both tins on the same shelf in the oven and bake for 25 minutes, or until golden.
Once cooked, take out and place, with the parchment, onto a wire rack until cooled.
For the filling, cream together butter, vanilla extract and milk. Fold in icing sugar, then whisk until fluffy. Don't over whisk it, as it will become sloppy.
On one of the halves of cooled sponge, spread a generous layer of jam and on the other most or all of the butter icing.
Sandwich together and sprinkle with a little icing sugar to decorate.