Preheat oven to 180ºC . Coat a dozen cup bundt pan with nonstick spray. In a large mixing bowl, beat margarine and sugar together with electric beaters. Add buttermilk, poppy-seed filling, egg substitute, vanilla and sour cream. Beat until well-combined. Add flour, baking soda and salt and stir with a spoon until just combined. In a separate bowl, beat egg whites until stiff but not dry. Fold into batter. Pour batter into prepared pan and spread evenly.
Bake in preheated oven about 1 hour and 10 minutes, or until toothpick inserted in center comes out clean. Cool in pan 10 minutes, then remove to wire rack to cool. Dust with powdered sugar, if desired.
Makes 16 or more servings.
Each serving contains approximately:298 calories, negligible cholesterol, 9 g fat, 334 mg sodium