Mix the light butter, brown sugar substitute, and granulated sugar substitute together until creamy.
Add the egg, egg whites, and vanilla and mix again.
Add all the dry ingredients and mix until a thick dough forms.
Fold in the Lily’s chocolate chips and let the dough sit for 5–8 minutes.
Press half the dough into a parchment-lined 8x8 pan, add as many marshmallows as you want, then cover with the remaining dough.
Top with extras if using.
Bake at 350°F for about 20–25 minutes, until the edges and top are golden.
Let cool completely before slicing so it firms up and gets crunchier.
