In a small bowl, combine the ground flaxseed and water. Let it sit for 5 minutes.
In a large bowl, add the softened vegan butter and brown sugar. Beat with a hand or stand mixer until creamy. Add the flaxseed mixture, pure vanilla extract, cornstarch, baking soda, and salt. Mix using the hand or stand mixer until combined.
Cut the dark chocolate bar into chunks. Cut the mini marshmallows in half and cut the graham crackers into rough chunks approximately the same size as the chocolate.
Fold in the marshmallows, chocoalte chunks and graham cracker pieces.
Scoop 2 tablespoons of dough and loosely form it into a ball. Arrange the balls on a lined cookie sheet and cook at 350 degrees for 13-14 minutes or until they are golden around the edges. Let cool for 10 minutes and then transfer them to a cooling rack.
The cookies will seem soft when you take them out of the oven, but they will firm up as they cool.