banana nut muffins

I've been wanting to make banana nut bread for awhile, but I didn't have any shortening, which my recipe calls for, so I found this recipe on allrecipes.com...

(I only made two minor changes -- I added about a teaspoon of vanilla to the wet mixture and added about a cup of chopped walnuts to the batter at the end.)

Banana Crumb Muffins
1 1/2 cups all-purpose flour
1 tsp. baking soda
1 tsp. baking powder
1/2 tsp. salt
3 bananas, mashed
3/4 cup white sugar
1 egg, lightly beaten
1/3 cup butter, melted
1/3 cup packed brown sugar
2 tbs. all-purpose flour
1/8 tsp. ground cinnamon
1 tbs. butter

Preheat oven to 375 degrees F. Lightly grease 10 muffin cups, or line with muffin tins. In a large bowl, mix together 1 1/2 cups flour, baking soda, baking powder, and salt. In another bowl, beat together bananas, sugar, egg, and melted butter. Stir banana mixture into flour mixture just until moistened. Spoon batter into muffin cups. In a small bowl, mix together brown sugar, 2 tbs. flour, and cinnamon. Cut in 1 tbs. butter until mixture resembles coarse cornmeal. Sprinkle topping over muffins. Bake in preheated oven for 18-20 minutes, until a toothpick inserted into the center of a muffin comes out clean.