Preheat the oven to fan oven 160C, 180C, gas mark 6. Line 2 x 20cm sandwich tins with baking parchment.
Use an electric whisk to beat together the butter or margarine, sugar, vanilla and salt until pale and soft, it may take several minutes to become paler. With the whisk running gradually add the beaten egg, whisking between additions until you have a smooth, stiff mixture. If the mixture splits add a tsp of the Homepride self-raising flour to bring it back together. Add the flour and fold in carefully. Spoon into the prepared tins and bake for 20-25 minutes until golden and risen – they should spring back when touched gently.
Leave in the tins for 5 minutes before turning out onto wire racks to cool completely.
For the filling; beat together the butter, icing sugar and vanilla until smooth. Spread one cake with the butter icing, top with the jam and lay the second cake on top. Dust with icing sugar before slicing.