In a large bowl, dissolve yeast in warm water. Add the milk, sugar, salt, egg, shortening, and 1-1/2 cups flour; beat until smooth. Add enough remaining flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch down. Turn onto a lightly floured surface; roll into a 30×20-in. rectangle. In a bowl , combine the butter, brown sugar, eggs, vanilla, lemon extract if desired and nuts. Stir in enough milk to achieve spreading consistency. Spread over rectangle to within 1 in. of edges.
Roll up jelly-roll style, starting with a long side; pinch seams and ends to seal. Place on a greased baking sheet; shape into a tight spiral. Cover and let rise until nearly doubled, about 1 hour.
Bake at 350° until golden brown, about 35 minutes. Remove from pan to a wire rack to cool. If desired, combine confectioners’ sugar and enough milk to make a thin glaze; brush over roll.
