Heat oven to 200C/180C fan/gas 6.
Oil a 25cm square or 20 x 30cm rectangular cake tin or baking dish.
Beat butter, sugar, flour, eggs and vanilla extract together until smooth.
Spread jam over the base of the tin
Dollop over the cake batter and spread evenly.
Bake 20-25 mins until the sponge is light golden and risen, and a skewer inserted into the middle comes back clean.
Serve warm with custard, cream or ice cream.
