Preheat the oven to 350°F (180°C).
Make the flax egg by mixing the flaxseed meal with the water. Let it sit for 5 minutes.
Melt the vegan butter. It should be melted but cooled. To do so, microwave it for 15 second on defrost function (repeat if necessary). If melting it on the stove, let it cool down for 10 to 15 minutes.
In a large mixing bowl, combine the oat flour, almond flour, baking soda and salt with a whisk.
Form a well in the middle and add in the flax egg, melted and cooled vegan butter, coconut sugar and vanilla extract. Start mixing with a whisk.
Pour in the plant-based milk and continue mixing with a rubber spatula.
Fold in the chocolate chips.
Scoop 4 balls of cookie dough with a 2-tablespoon cookie scoop onto a parchment-lined baking sheet. Add another ball of cookie dough onto the first while pressing it flat. Wet your hands with water and slightly press down the second ball of dough (see step-by-step pictures in post above).
Bake the cookies for 10 minutes.
Sprinkle some flaky sea salt on top. Let them cool on the baking sheet for 10 minutes. Then, transfer them to a cooling rack and let them cool for 2 hours. The texture improves tremendously as the cookies cool down.