Grease pyrex square or rectangular casserole with butter. Cut 8 Twinkies in thirds, LENGTHWISE, and put one layer on the bottom of the casserole. Beat egg whites, with the cream of tartar and sugar, adding vanilla. Melt chocolate chips in the top of a double boiler. Add egg yolks to chocolate, slowly, continuing to stir over boiling water. Fold chocolate into egg whites. Spread over Twinkies, then sprinkle with about half of the nuts. Layer on more twinkies, more chocolate, more nuts. Continue layering. Top with whipped cream and a single whole Twinkie. Chill and serve.