Crush the graham cracker squares in a food processor or place in a gallon size bag and roll with a rolling pin.
Add sugar and butter and mix into the crushed graham cracker.
Press 1 tablespoon graham cracker mixture into the bottom of a cupcake liner. I put 1 T in and then pressed down with 1/4 cup measuring cup. You have the option to bake this at 350 degrees for 8 minutes so that it can harden. I skipped this part and it turned out great!
In a separate bowl, mix together egg whites, condensed milk, lime juice, lime zest, and greek yogurt. Mix well.
Fill the cupcakes about 3/4 of the way full with the mixture. Bake for 12-15 minutes and cool.
Top with whip cream and garnish with lime if desired. Store in the refrigerator.