Pre-heat oven to 170C Fan and spray a 24cm square cake tin evenly with the low fat cooking spray. Then line the tin with greaseproof paper.
Sieve the bicarbonate of soda and self-raising flour into a large bowl. Then stir in the sugar, vegetable oil, vanilla essence, chopped walnuts, coffee mixture and yogurt.
In a separate bowl, beat the egg whites to soft peaks and then gently fold them into the cake mixture.
Spoon the cake mixture into the cake tin using the back of a warm spoon to level the surface flat.
Bake for approximately 30 – 35 minutes in the centre of the oven. Check the cake is cooked through by inserting a skewer into the centre and ensure it comes out clean. Remove from oven.
Allow the cake to rest in the tin for a couple of minutes and then turn out the cake onto a wire cooking rack, remove the greaseproof paper and allow to cool completely.
To make the coffee icing topping:-
Thoroughly mix together the coffee and water mixture with the sieved icing in a bowl until smooth.
Spread the coffee icing mixture evenly over the surface of the cake using a palette knife.
Decorate the top with the snapped walnut pieces and once the icing has set cut the cake into 12 slices.