Pre-heat the oven to 180° C.
Grease and line a 7 inch square cake tin. In a large mixing bowl, combine the butter, sugar, flour, eggs and vanilla extract and beat with an electric hand-held whisk for 1-2 minutes until smooth. Spoon into the prepared cake tin and bake for 25-30 minutes, or until a cake tester comes out clean when inserted into the centre of the cake.
Remove from the oven when ready and allow to cool in the tin for 5 minutes before turning out onto a wire rack to cool completely. Once cool, heat the raspberry jam gently in a saucepan for 2 minutes over a low heat, stirring occasionally.
Use a sharp bread knife to level off the top of the cake. Spread the jam on top, allowing it to sink in a little. Cut into square portions and arrange on serving plates. Garnish with a few raspberries and a light dusting of icing sugar.