Preheat the oven to 190C/375F/Gas 5. Grease and line two 18cm/7in sandwich tins.
Sift the flour and baking powder into a bowl.
Cream the margarine, sugar and golden syrup together in a bowl until light and fluffy. Add a little flour then a little milk and repeat until all of the flour and milk has been added and the mixture is smooth and well combined. (Add a little extra milk or water if the mixture is too stiff.)
Divide the mixture between the two sandwich tins. Bake in the oven for 20-25 minutes, or until risen and golden-brown.
Remove the cakes from the oven and set aside to cool slightly. Remove the cakes from the tins and set aside to cool completely.
To serve, sandwich the cakes together with the jam.