Chocolate Banana Muffins

1/2 cup butter, softened
1 cup sugar
2 eggs
1 1/2 cups flour
3 Tbs. cocoa
1 tsp. baking soda
1 tsp. salt
1/2 tsp. cinnamon
1 tsp. vanilla
1 cup mashed banana
1/2 cup sour cream
1/2 cup finely chopped walnuts
1 cup chocolate chips

*Cream butter and sugar. Add eggs. Add dry ingredients and mix well. Add vanilla, banana, sour cream, nuts and chocolate chips. Pour into muffin tins and bake at 350 degrees for 15-20 minutes.