For topping, in a medium bowl stir together powdered sugar and whipping cream until smooth. Divide mixture between two 2-quart rectangular or square baking dishes. Sprinkle the 1 cup chopped nuts evenly over mixture in baking dishes; set aside.
For filling, in a small bowl stir together brown sugar, flour, and cinnamon; set aside.
On a lightly floured surface, roll each loaf of dough into a 12x8-inch rectangle (if dough is difficult to roll out, let it rest a few minutes and try again). Brush dough rectangles with melted butter. Sprinkle filling, 1/2 cup of the chocolate pieces, and the 1/2 cup chopped nuts evenly over each rectangle. Roll up each rectangle, starting from a long side. Pinch dough to seal seams. Slice each rolled rectangle into 6 equal pieces. Arrange six pieces, cut sides down, in each prepared dish. Cover; let rolls rise in a warm place until nearly double in size (30 to 45 minutes). Use a greased toothpick to break any surface bubbles.
Preheat oven to 375 degrees F. Bake in the preheated oven for 30 to 35 minutes or until sides of rolls are brown and center rolls do not appear doughy (do not underbake*). If necessary to prevent overbrowning, cover loosely with foil during the last 15 to 20 minutes of baking. Cool in dishes on wire racks for 5 minutes; remove from dishes.
In a small saucepan combine the remaining 1/2 cup chocolate pieces and the shortening. Cook and stir over low heat until chocolate is melted and smooth; drizzle over cinnamon rolls. Serve warm.
The topping will turn to a caramel color as the rolls bake. If you pick up the baking dish, you should see the topping underneath the rolls appear caramel colored when the rolls are done.