Preheat the oven to 350 degrees F (180 degrees C) and line a baking sheet with parchment paper.
Prep the flax seed egg by whisking together ground flaxseed and water. Let it set in the fridge for at least 15 minutes, or up to overnight.
In a large mixing bowl, beat the softened vegan butter with the light brown sugar and caster sugar until smooth and fluffy.
Add the flax egg and vanilla extract. Mix to combine.
In a separate bowl, whisk together gluten-free all-purpose flour, xanthan gum, salt, and baking soda.
Add the whisked dry ingredients and the rolled oats to the wet mixture. Fold everything together until combined. The mixture will be thick and sticky.
Fold in vegan chocolate chips.
Roll the mixture into 2-inch balls (about 60g each). Spread out on prepared baking sheet. Flatten each cookie slightly with your hand. You may need to make 2 batches.
Bake for 12 to 15 minutes, or until slightly browned on top.
Transfer cookies to a wire rack to cool. The cookies will be very soft in the middle when first removed from the oven.