Combine yeast, sugar, and buttermilk. Add melted butter and stir until mixture is dissolved. Sift together flour, salt, and soda and add gradually to yeast and sugar mixture. Stir until smooth. Form dough into a ball and place in buttered bread bowl, turning so entire surface is greased. Cover bowl with a clean cloth and let dough rise in warm place until doubled in bulk. Punch down dough and form into desired shapes. (Put single balls of dough into greased muffin pans, filling each hole 1/3 full, or make larger balls and place on greased baking sheet.) Brush with egg white and sprinkle with poppy seeds. Cover and let rise again in warm place until doubled in bulk. While dough is rising preheat oven to 400 degrees F. Bake 20 minutes.