You can use any shape cookie cutter to make these sandwiches. Choosing a smaller shape yields more cookies! These cookies are free of dairy, egg, soy, peanuts, and tree nuts.
In a small bowl, mix the flax seed meal and water together; set aside. In a medium bowl, whisk together the flour, cocoa powder and salt; set aside. With a mixer on medium speed, cream the margarine and powdered sugar together until fluffy. Beat in the flax seed meal and water mixture and vanilla extract. With the mixer on low speed, gradually add the flour mixture until fully incorporated. Form dough into a ball and place on a piece of plastic wrap (dough will be sticky). Chill in the refrigerator for at least 2 hours (I chilled mine for 2 days!)
Preheat oven to 325 degrees. Spray cookie sheet with cooking spray. Divide dough in half. On a floured surface, roll dough to 1/8" thickness (you will likely need to flour your rolling pin to prevent sticking as well). Using a cookie cutter, cut out shapes and place on the baking sheet. Gather scraps, flour surface again and continue to roll out dough until all is used. If dough becomes too soft, return to the refrigerator to firm.
Bake cookies in 325 degree oven for 12-14 minutes turning cookie sheet halfway through. Remove from the oven and place on a cooling rack or paper towels to cool.
In a mixer on medium speed, cream together the margarine and powdered sugar. Add the mint extract (or other flavoring) and food coloring to desired flavor and color.
To decorate: Using a cake decorator, pipe the frosting onto the bottom of a cookie. Place another cookie on top and lightly press until filling spreads to the edge of the cookie. Alternatively, use a spreader to spread frosting on the bottom of cookie; place another cookie on top of the filling and press lightly until the filling is at the edge of the cookies. Continue until all of the sandwiches have been made.