Melt the vegan butter in the microwave and make the flax “egg” in a small bowl by mixing the flax and water together. Set aside at least 10 minutes for butter to cool and “egg” to thicken. In the meantime, gather the other ingredients.
Preheat the oven to 325˚F (160˚C) and line a 8x8-inch baking pan with parchment paper.
In a large bowl, mix together the melted butter, brown sugar, granulated sugar, flax egg, and vanilla extract until evenly combined.
Add the oat flour, rolled oats, baking soda and salt and mix until uniform. Stir in the chocolate chunks.
Spread batter evenly in the prepared pan and top with extra chocolate chips if desired. Bake for 30-35 minutes, until edges are golden brown and cracks form on the top. It may seem underdone when it first comes out of the oven, but it firms up quickly as it cools.
Once done, remove from the oven and let cool for at least 30 minutes before serving. The longer it cools, the cleaner it will slice.
Lift out of the pan using the edges of the parchment paper and slice into 16 bars with a sharp knife.