Preheat oven to 375 F degrees.
Line a muffin tine with paper cups or spray with non-stick cooking spray.
Beat the butter, sugar substitute and regular sugar together until crumbly.
Add in the egg whites while beating.
Stir in the lemon peel. Combine well.
In a separate bowl whisk together the flour and baking soda.
Add a little of the flour mix to the batter, then a little of the yogurt. Stir well after each addition then continue alternating between the flour and yogurt.
Fill muffin cups 2/3 full with the batter.
Bake about 18 minutes. A toothpick removes clean from the center when done.
Cool 5 minutes in the pan, then place on a rack to finish cooling.