Preheat the oven to 160°C and grease the ovenproof dish well with the little extra reduced-fat spread.
Put 1 tbsp coffee granules in a small bowl or cup and add 1 tsp boiling water. Stir until the coffee granules have completely dissolved and leave to cool.
Put the flour, reduced-fat spread, granulated sweetener, caster sugar, baking powder, cooled coffee mixture, eggs and vanilla extract in a medium mixing bowl and beat together for 1- 2 minutes with an electric whisk until light and creamy. You can use a wooden spoon instead of a whisk, but it will take more effort.
Use a rubber spatula to scrape the mixture from the mixing bowl, into the greased ovenproof dish and level the surface with a knife. Bake in the preheated oven for 20-25 minutes, until risen and spongy. To test if the sponge is ready, insert a small sharp knife into the centre; when the sponge is cooked the knife will come out clean. Leave the sponge in the dish to cool.
Put 1 tsp coffee granules in a small bowl or cup and add 1 tsp boiling water. Stir until completely dissolved and leave to cool.
Sift the icing sugar into a small bowl and stir in the coffee mixture until completely smooth and runny enough to drizzle. Drizzle the icing over the cooled sponge and sprinkle with chopped walnuts. Allow the icing to set and cut into 16 squares.