Mix together the ground flaxseed and water and let sit until slightly thickened, about 5-7 minutes.
Cream together the butter and sugars until light and fluffy, about 5 minutes.
Add the flax and vanilla and mix well. The batter may look broken or lumpy, but that's ok!
Sift together the flour, protein powder, baking soda, salt, and cornstarch and add into the mixing bowl all at once. Mix until flour is just incorporated.
Add the applesauce and mix until just combined. The dough should form into one big ball and pull away from the sides of the mixing bowl. If the dough looks super crumbly or dry, add an additional tablespoon of applesauce.
Fold in the chocolate chips.
Add the chocolate hazelnut spread all at once, then quickly turn the mixer on and off a few times so that the spread is just starting to get incorporated. You can also fold it in by hand, but it's a little harder to do.
Scoop the dough onto a parchment-lined baking sheet, then chill in the fridge until cold, about 10-20 minutes. While the dough is chilling, preheat the oven to 350F.
Sprinkle the cookies with cinnamon, then bake 13-15 minutes until the cookies are set and slightly golden around the edges.
Note: These cookies will puff up in the oven, so I like to smash them. This step is totally optional, but I think it gives the cookies a better texture. You can flatten the dough before you put the cookies in the oven, but I like to bake the cookies, let them cool for 5 minutes, and then flatten them with the back of a flat spatula.