Step #1 Preheat the oven to 350°F Grease 12 large size muffin c with butter or cooking spray or line with paper muffin wrappers.
Step #2 Wash & dry the lemon.
Step #3 Cut into quarters & remove the seeds.
Step #4 Process the quartered lemon (all of it) in a food processor/blender, or mixer, fitted with a metal blade for 1-min or more until it is completely ground up.
Step #5 Scrape the lemon into a small bowl, & set aside for later use.
Step #6 Using a mixer fitted with a paddle attachment, cream the butter In a large-ish bowl on medium high speed for 1 min.
Step #7 Scrape down the sides of the bowl.
Step #8 Add the Splenda & beat on medium high until light & fluffy.
Step #9 Add the egg whites all at once & beat on medium speed for 1 min.
Step #10 Add the vanilla & beat for about 30-seconds.
Step #11 Mix together the dry ingredients.
Step #12 Beat one third of it into the creamed mixture just until incorporated.
Step #13 Add half the yoghurt & do the same.
Step #14 Repeat, ending with dry ingredients.
Step #15 Gently stir in the the lemon.
Step #16 Spoon batter into prepared muffin tins.
Step #17 Bake for 30-mins until they spring back when carefully touched, or a toothpick inserted in the center comes out clean.
Step #18 Let cool in the pans for 5 mins, then remove to a wire rack.
Step #19 These are at their very best served warm, but if wrapped tightly in plastic wrap once cooled, they will still taste great for up to 3 days.