Preheat oven to 350ºF. Line muffin tins with 18 mufifn liners and set aside.
In a medium-sized mixing bowl, pour the cake mix. Pour the diet soda in slowly to the cake mix. Stir together with a whisk until all lumps are gone.
Scoop batter into muffin liners, filling about ¾ full. Bake for 18-20 minutes, until a toothpick comes out clean. Let cool on a cooling rack.
In a medium-sized mixing bowl, cream together butter, powdered sugar, lemon juice and vanilla. Blend together until all combined and smooth, adding more lemon juice/milk or powdered sugar to get the desired thickness.
Frost cooled cupcakes and serve. Enjoy!