Preheat oven to 350 °F. Grease and dust a 10-cup bundt pan with dry breadcrumbs, tapping out excess.
Beat butter until light and fluffy. Add 1 ¼ cups of sugar to butter, ¼ cup at a time, beating after each addition until fluffy. Beat in lemon zest and vanilla.
In a separate bowl, sift the flour, cornstarch and salt. Add flour mixture, starting and finishing with flour, alternately with half-&-half, blending on medium-low speed after each addition.
In a clean bowl, whip egg whites with remaining ¼ cup sugar until they hold a stiff peak when beaters are lifted. Stir a third of the whites into the cake batter, then fold in remaining whites. Quickly fold in rhubarb and scrape into prepared pan. Bake for 60 to 70 minutes, until a tester inserted in the center of the cake comes out clean. Cool for 20 minutes in the pan, then turn out onto a rack to cool completely. Dust with icing sugar before serving.
Cake slices best with a serrated knife.