In a large bowl, dissolve yeast in 1/3 cup warm milk & allow to become frothy, about 5-10 minutes. With an electric hand mixer, beat together sugar, salt, warm melted butter, egg & vanilla. When yeast is ready, Combine with egg mixture. Add flour, 1 cup at a time to wet mixture. Stir well after each addition; dough should become smooth & elastic. It will not be firm enough to knead into a ball, more like thick batter. Cover loosely with plastic wrap & set in a warm, draft-free place to rise for an hour or until doubled in bulk.