Gently knead the white fondant until pliable. Take a small handful at a time and gently roll out to 1/4" thick using the rolling pin. If the fondant gets sticky, use the powdered sugar on your rolling pin and surface to prevent sticking.
Once the fondant is rolled to about 1/4" use the square cookie cutter to cut the fondant into small squares.
Set the fondant squares on the baking sheet for 30-60 minutes to set.
After the fondant hardens a bit spread a small amount of frosting on the back of the square and place on top of the graham cracker.
Use the edible food marker to make 4-5 dashes on on all sides of the square and then a small triangle in the middle.
Use the toothpick to place a small amount of frosting on the back of the candy heart and then affix onto the 'envelope flap'.
Repeat with all 10 graham crackers.
Courtesy of Mom Foodie - Valentines Treat Love Letters find it at https://momfoodie.com/valentines-day-treats/