Why does my walnut cake fall? The recipe uses 3 eggs for volume, separated, yolks and sugar whipped over hot water until triple in volume, whites whipped to stiff peaks. Ground walnuts, 2 tbsp breadcrumbs, 1/4 c butter are all folded in. baked at 350 for 40 minutes.

During baking it looks great, rises and is perfection. On cooling the center starts to fall, then the sides turn into the center.

How do you stop it falling? Am i overwhipping?
This "cake" is practically a souffle, and falls for the same reason a souffle does: it has nothing to keep it tall. It first rises because of the steam created during baking, but once the steam production is over, it deflates. Unlike real cakes, which include flour, there is nothing to create a persistent structure here. 

There is nothing to prevent a souffle from falling when it has cooled, it is like expecting a punctured baloon to stay inflated. I would suggest using a different recipe. Alternatively, you can decide that you are OK with a moist, fudgy cake without much rise. To avoid a mishappen shape from first rising too much and then falling, whip the eggs less and combine them more thoroughly with the rest of the ingredients, while still keeping some air in.