Sweet Potato Casserole

I'm not a huge sweet potato fan, but I had my first exposure to sweet potato casserole last Thanksgiving and I actually enjoyed it.
This is my fiance's Aunt Pat's recipe from last year, I made it this year and it turned out great.

ingredients
4 huge sweet potatoes
2 yams
1C sugar
4 eggs, lightly beaten
1C milk
1 stick butter, melted
1 tsp salt
1 tsp vanilla

for topping
1C brown sugar
1C walnut pieces
2/3C flour
2/3C butter

directions

1. Preheat oven to 425 degrees.
2. Roast the whole sweet potatoes and yams on an aluminum foil covered baking sheet at 425 degrees until soft, about 45-60 minutes. Peel and cool until ready to use. 
3. Heat oven to 350 degrees. 
4. Grease a 2qt casserole dish. 
5. In a medium bowl, stir together yams, sweet potatoes, sugar, eggs, milk, butter, salt, and vanilla  until mashed and smooth. 
6. Put mixture into greased dish. 
7. To make topping - in a small bowl, mix brown sugar, walnuts, flour, and butter with you (clean!) fingers until crumbly. 
8. Sprinkle over top of dish. 
9. Bake uncovered for 30-35 minutes until browned on top. 
10. Let stand 10 minutes before serving. 
11. Enjoy!