Preheat the oven to 180°C, or Mk 5. Turn off the fan in a fan oven if possible, otherwise put the cake on the highest shelf. Put some water in a cake tin or roasting tin and set in the bottom of the oven. Prepare the sandwich tins by lining with baking parchment.
Cream together the Billington’s caster sugar and sunflower spread with a wooden spoon or Kenwood electric hand whisk, until pale and fluffy. Add the eggs in 3 parts, beating well after each addition so that the egg is well incorporated. Add the Vanilla Extract. Dissolve the coffee in the boiling water and stir that in.
Mix in the chopped walnuts and fold in the flour lightly until the mix is even. Spoon this into the prepared tins and bake for 25 minutes. Set aside on a wire rack to cool.
Meanwhile, make the topping and filling. Dissolve the coffee in the water. Cream together the sunflower spread and Billington’s icing sugar. Stir in the coffee. Refrigerate until ready to serve.
To assemble, put a dab of filling in the centre of the serving plate, and top with one of the cakes. Spoon the filling all over the base and top with the other cake. Decorate with the walnut halves.