Preheat the oven to 160°C and grease a 20 x 22cm ovenproof dish well, using the extra reduced-fat spread.
Place the flour, reduced-fat spread, white granulated sweetener, caster sugar, baking powder, eggs and vanilla extract in a medium mixing bowl. Use an electric whisk to beat together for 1-2 minutes. Alternatively you can use a wooden spoon, but it will take more effort.
Use a rubber spatula to scrape the mixture from the mixing bowl and into the greased ovenproof dish. Level the surface with a knife and place in the preheated oven. Bake for 20-25 minutes until risen and golden all over. When you insert a small, sharp knife into the centre, it should come out clean.
Place the jam in a small bowl and stir until smooth. Push the jam through a small sieve to remove any bits, then spread the jam all over the top of the warm sponge.
Leave the sponge to cool completely before decorating. You can leave the sponge in the dish and pipe a spider web on top using black writing icing. Then add spider cake decorations and cut into 16 squares. If you'd prefer to serve the whole sponge on a large plate, loosen each square with a small palette knife and lift out of the dish one by one. We found this was easier than turning out the whole sponge. Place the sponge squares on a large serving plate and rearrange into the whole sponge and decorate as above.