Using a stand mixer, beat butter on low speed in a large mixing bowl until light. Gradually and sugar and continue to beat until mixture is light and fluffy.
In a small bowl beat egg, milk, and vanilla extract with a fork until mixed.
Add egg mixture to creamed butter and sugar and continue to beat until all is mixed.
Turn mixer to low speed and add the flour. Mix just until the flour is incorporated.
Gather dough into a ball and flatten into a disk. Wrap with cling wrap and refrigerate for at least 1 hour.
After dough is well chilled and you are ready to make the cookie cups, preheat the oven to 350°F/177°C.
On a lightly floured surface roll out the sugar cookie dough 1/8″ thick and cut it with a flower cutter. Gather scraps and reroll as needed.
Place the flower cookies in a 24-cup miniature muffin pan. Place one flower in every other muffin cup to give the cookies enough room so that they do not touch each other. Gently press the cookie dough into the pan so the cookie lines the inside of each muffin cup and the petals fan out on top of the pan. A 24-cup pan will hold 12 cookies.
Bake the cookies for 8 to 10 minutes at 350°F/177°C. The edges will just begin to have a tinge of brown.
While the cookies bake, unwrap the thin peanut butter cups and cut the marshmallows in half crosswise using a knife or scissors.
Remove the cookies from the oven and immediately top each cookie with a peanut butter cup. After all of the cookies are topped with the peanut butter cup, place a marshmallow cut-side down on top of the peanut butter cup.
Set oven broiler to low and broil the cookies just until the marshmallows are toasted. Watch carefully and turn pan as needed as marshmallows toast. This only takes a few seconds, so watch the cookies continuously.
Remove the cookies from the broiler. Let cookies cool 2 to 3 minutes before removing them from the pan.
Repeat cutting flower cookie cups with the remaining dough. Bake and top as directed for a total of 24 cookies.
Serve and Enjoy!
♥ The sugar cookie dough can be made up to 3 days in advance and kept in the refrigerator until you are ready to bake the s'mores cookie cups.♥ The flower cookie cutter I used was 3 3/8" in diameter. This is just the right size for the cookie dough to easily fit inside the cup of a miniature muffin pan and have the petals fan out on top of the pan.♥ The cookie cups can be stored in a single layer in a lidded container. ♥ These reheat beautifully! Just a few seconds in a microwave will warm these nicely or you can warm them in a 350°F/177°C oven for a few minutes.