In a large mixing bowl add the three cans drained sweet potatoes, three eggs, entire can of sweetened condensed milk, 1 teaspoon of cinnamon, 1 cup dark brown sugar and 2 teaspoons vanilla extract. With mixer or food processor, combine all until creamy and fluffy -- some small chunks may remain.
In large EZ foil lasagna pan (can throw away or just leave it there!)or medium roaster pan (need at least 2 - 2 1/2 inches deep) coat with cooking spray or grease lightly with oil of your choice. Pour in sweet potato mixture - spread edge to edge.
In seperate mixing bowl, add two sticks of butter softened, 1 1/2 cup of dark brown sugar, 1 teaspoon cinnamon, and walnuts CHOPPED. Mix together until you have a soft mixture. Spread evenly on top of sweet potato mixture.
Bake at 350 degrees for 45 minutes uncovered, or until top is golden brown crispy. If top starts to darken up too fast, lay a piece of aluminum foil over it loosely...Let set for at least 30 minutes before serving so it can set up. If you are taking on the run, do not TOTALLY cover pan -- loosely put on aluminum foil so top does not get soggy
This will serve quite a few at your holiday feast!