Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
In a large bowl, cream together softened butter and sugar substitute until light and fluffy.
Add the egg, egg yolk, and vanilla extract; mix until fully combined.
Gently fold in the sugar-free Cool Whip until smooth and airy.
In a separate bowl, whisk together flour, baking powder, and salt.
Gradually add dry ingredients to wet ingredients, mixing just until combined.
Fold in optional mix-ins if using.
Scoop dough (1–1½ tablespoons) and place on prepared baking sheet, spacing 2 inches apart.
Optional: Roll dough balls lightly in powdered sugar substitute for a crackled finish.
Bake for 10–13 minutes, until edges are set and tops are slightly cracked.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.