Mix 1/2 cup butter, sugar and salt in a large bowl. Add egg. In a separate bowl, combine yeast flour; mix thoroughly. Add flour mixture to butter/sugar mixture, then add warm water. Mix well and until soft smooth dough forms. Turn out on lightly floured surface. Knead until smooth. Cover and let dough rise for 15-30 minutes in a warm, draft free location.
Punch down dough and let rest for five minutes. Roll into a 14 x 8-inch rectangle (approximately) on a lightly floured surface. Brush each with softened butter for filling. Combine cup sugar, cornstarch and cinnamon. Sprinkle over buttered dough. Roll up from long side. Cut into 12 cinnamon rolls.
Place cinnamon rolls in 13x9 inch pan that has been lightly brushed with butter. At this point you can place rolls in the fridge to bake the following morning. Or, preheat your oven to 325 degrees F and let the dough rise for 30 minutes, then bake.
If placing rolls in the fridge, remove from fridge ONE HOUR prior to the time you will start baking them. Let them rest in a warm, draft free location for 40 minutes, then start preheating oven to 325 F.. Once oven is heated, bake rolls for approximately 20 minutes (or until lightly browned).