Chocolate Chip Muffins

1/2 cup butter, softened
1 cup sugar
2 eggs
1 cup (8 oz.) plain yogurt
1 tsp. vanilla
2 cups flour
1 tsp. baking soda
1/2 tsp. baking powder
1/2 tsp. salt
3/4 cup semisweet chocolate chips

Topping:
1/4 cup semisweet chocolate chips
2 T. brown sugar
2 T. chopped walnuts
1 tsp. cinnamon

Cream butter and sugar. Add eggs, one at a time, beating well after each addition. Add yogurt and vanilla; mix well. Combine the flour, baking soda, baking powder and salt; add to the creamed mixture just until moistened. Fold in chocolate chips. Fill muffin cups two-thirds full.