Dissolve the yest in the warm milk in a large bowl. Add sugar, margarine, salt, eggs and flour, mix well. Knead the dough into a large ball, using your hands dusted lightly with flour. Put in a bowl, cover and let rise in a warm place about 1 hour or until the dough has doubled in size. Roll the dough out on a lightly floured surface, until it is approximately 21 inches long by 16 inches wide. It should be about ¼ inch thick. Preheat oven to 400° F. Combine brown sugar and cinnamon in a bowl. Spread softened butter on rolled out dough. Sprinkle with sugar-cinnamon mixture. Working carefully, from the long edge, roll the dough down to the bottom edge. Cut the dough into 1 ¾ inch slices, and place in a lightly greased baking pan. Bake for 10 minutes or until light golden brown. While the rolls are baking, combine icing ingredients. Beat well with an electric mixer until fluffy. When rolls are baked, spread generously with icing.