In food processor, puree beans until coarse. Add in sugar, milk, egg whites, margarine and vanilla and puree until smooth, scraping down sides a few times.
In a large bowl whisk together flour, cocoa, wheat germ and baking powder. Pour bean mixture over flour mixture. Stir in walnuts to combine. Scrape batter into parchment paper lined 8-inch (1.5 L) square baking pan, smoothing top.
Bake in 350F (180 C) oven for about 22 minutes or until cake tester inserted comes out clean. Let cool on rack.