In the bowl of a stand mixer fitted with a paddle attachment, add the butter, sugars, and salt. Cream until they're until well-incorporated and airy.
Add the egg and vanilla.
Beat until the mix resembles the texture of a smooth buttercream.
Add the flour, baking soda, and baking powder and mix on a low until just combined.
Using your hands, fold in oats, pecans, flax seeds, and shaved dark chocolate.
Roll into balls (~ 3 oz in size), then chill dough for at least 30 minutes. (The longer you chill it, the less it will spread during cooking. (I chill mine at least 24 hours, preferably 72 hours)
Preheat oven to 350° F.
Bake for 15 to 16 minutes, then let them sit on the pan for 10 minutes before transferring to a cooling rack.
Note: they won't look done when you take them out of the oven, but the residual heat from the pan will give them just the right amount of gooey-ness as they rest.