In a mixing bowl, beat butter with an electric mixer on low speed or by hand until creamy. Then, blend egg.
Slowly add mix and blend until cookie is formed. Gather dough ball and form into small flat disks.
Wrap dough in plastic wrap and freeze for 20 minutes or until dough is firm.
Preheat oven to 350 degrees. Remove dough from freezer. Form dough into half-tablespoon size balls and place on parchment-lined cookies sheet. Bake for 12-14 minutes
Cool on cookie sheet for 5 minutes then move to wire rack to cool.
Melt the dark chocolate in the microwave in 30-second increments. Continue until completely melted.
Heat heavy cream until lukewarm. Carefully whisk heavy cream in the chocolate mixture until fully combined and has a smooth consistency.
Place marshmallow puff in a piping bag with large tip.
Using a cookie rack, place cookie and pipe out a dollop of marshmallow on top of the cookie. Pour chocolate over cookie until completely covered. Tap rack to take off excess chocolate.