Preheat oven to 350 degrees Fahrenheit. Line baking sheet with parchment paper or a baking mat.
Make flax egg: Mix flaxseed and water and place in the fridge for 5 minutes to set.
In a large mixing bowl, use a hand mixer or stand mixer to cream together sugars and coconut oil until nice and fluffy.
Add flax egg and vanilla extract to the mixture and beat a few times until just mixed.
In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.
Add 1/3 dry mixture to wet mixture and beat on low speed. Continue adding the dry mixture to wet until the dough is just mixed. The dough should be firm and pliable. If the dough is a little crumbly, add 1-2 Tablespoons of water or almond milk.
Using a rubber spatula, fold in chocolate chunks.
Use a cookie scoop to spoon dough onto a baking sheet. I like to gently roll mine with my hands and slightly flatten the tops.
Bake cookies for 10 minutes. Remove from the oven and let cool slighty before transferring to a cooling rack.
Coconut oil should be slightly soft (like room temperate butter). If it's too soft, you can place in the fridge to firm up.
Store cooled double chocolate cookies in a sealed container at room temperature for up to 5 days.
To freeze, individually wrap each cookie in plastic wrap before storing together in an airtight freezer bag. Keep in the freezer for up to 2 months.