In large bowl of mixer, cream butter and sugar until light and fluffy. Stir in sweet potatoes. Add eggs, one at a time, stirring well. Sift together flour, baking powder, salt, nutmeg, and cinnamon. Alternately add milk and dry ingredients to sweet potato mixture, stirring well after each addition.
Gently stir in pecans and lemon extract. Fill med. sized greased muffin tins about 2/3 full. Mix topping ingredients; sprinkle evenly over muffins. Bake 20 to 25 min., 400 deg. F oven.
1/2 cup sugar
1 Tablespoon cinnamon