Preheat oven to 350°F. Line a baking sheet with parchment paper for easy clean up.
First, make a flax egg: stir ground flax with 3 tablespoons hot water in a small dish and set aside.
In a medium bowl, cream peanut butter and maple syrup together with a hand mixer for a minute until light and fluffy. Add flax egg and vanilla, and mix until combined.
Add the rye flour, hemp hearts, salt, baking soda and cinnamon and continue mixing until just incorporated, do not over mix. If adding chocolate chips, stir them in by hand now.
Roll about a tablespoon of dough into a ball and then press into 2 inch (5cm) diameter cookie. Space cookies about 1 inch (2.5cm) apart.
Bake for 8-9 minutes until just firm around the edges and the top starts to crack.
Remove from oven and leave them on a baking sheet for 5 minutes before transferring them to a coolingÂ rack where they will continue to firm up.Store at room temperature for up to three days (if they last that long!) or freeze once fully cool.