Cooking eggs over low heat ensures that they will be creamy and tender. Grease and flour 10-inch tube pan. Add eggs, one at a time, beating between additions. In separate bowl, sift together flour, baking powder, baking soda and salt. With mixer on low speed, add flour mixture to butter mixture, mixing only until just combined.
Finish stirring with spatula to be sure batter is completely mixed.
Mix in walnuts, if desired. Spoon half of batter in prepared pan and spread out with knife. Spoon remaining batter in pan. Level out mixture and scatter remaining streusel on top. Bake 50-60 minutes in preheated oven or until toothpick inserted into cake comes out clean.