You have two options for the cookies;  you can either use a round cookie
 cutter to cut them perfectly round, or use the cookies as they are - 
which is a nice round cookie anyway.  Size is up to you as well.  
Larger, smaller, it doesn't matter.  Just use 2 cookies from your 
favorite recipe (I will put one here just in case you don't have one) 
and then sandwich them together with a bit of filling.  That's all there
 is to it!
Chocolate Chip Cookies
1/2 c butter 
1  c  packed brown sugar
3 T white sugar
1 large eggs
1 t vanilla
2 c flour
1/2 t  baking soda
1/2 t baking powder
1/2  t salt
1 1/2 c chocolate chips 
 
Filling
Half of a 7 oz. jar Marshmallow Fluff (creme)
1/4 c white, regular shortening
1/6 c powdered sugar
1/2 t vanilla
dash of salt
In  a bowl, with an electric mixer on medium speed, beat butter and 
sugars until well blended. Beat in eggs and vanilla until smooth,  
scraping down sides of bowl as needed. In another bowl, mix  flour, 
baking soda, powder, and salt. Stir or beat into butter mixture until  
well incorporated. Stir in chocolate chips. 
Drop dough, 2 inches apart, onto baking sheets, greased or on parchment 
paper. Bake  in a 375° oven until cookies are lightly browned, about 
10-12 minutes.  Bake them about 1-2 minutes longer if you think they may
 still be too soft.  Remember, the filling is going to make them very 
moist and soft after a couple hours.  A crisp cookie is good to start 
with!
Make the filling while the cookies are baking by mixing all in a bowl 
with an electric mixer until fluffy.  If it's a little dry, add just a 
teaspoon of warm water to it.  You want it to be stiff and not runny 
though.
When the cookies are done, remove them from the oven, let them set 2-3 
minutes and then remove to cool completely.  If you are cutting them to 
be perfect circles, use a cookie cutter and cut them out while still 
warm (!).  If you want until they are completely cool they may crack or 
crumble if they are very crisp.  When completely cool, spread a bit of 
filling on the bottom of a cookie and sandwich it together with a second
 cookie.  Let set at least an hour or two so the filling can soften the 
cookie.  They will be perfect after about 6 hours if you can wait that 
long.   *if you 'cut out' your cookies you will have scraps.  I save 
mine in a Ziploc in the freezer to crumble and use as a cheesecake base.