Grease and dust baking tin.
Mix sugar, coffee and water in a big pan. Heat till sugar dissolves. Remove from fire. Keep aside to cool.
Sieve flour with baking powder.
Beat egg whites till stiff peaks and keep aside.
Beat butter and egg yolks until creamy.
Add the sugar coffee syrup.
Stir in the sifted flour.
Fold egg whites into the mixture gently in two batches till well mixed.
Transfer to a greased and dusted pan. Level the top of the mixture with the help of spatula.
Bake at 180 degree centigrade for 40 minutes.
When it's baked transfer to a cooling rack.
For the sauce boil 1/2 cup of water with sugar, coffee and butter. Simmer for 2 minutes. Add cornflour paste and stir till thick and coats the back of the spoon nicely.
Prick cake with a fork. Pour sauce on the top and let it flow down the sides too.
Garnish it with walnut.
Coffee cake is ready.