Mix ground flax and water in a small bowl; let sit for 5 minutes to thicken.
Preheat oven to 350°F and line a 7x11-inch pan with parchment paper.
In a large bowl, whisk together melted vegan butter, granulated sugar, and brown sugar until combined.
Add the flax mixture and vanilla extract to the butter mixture; stir well.
Sift flour, cocoa powder, salt, and baking powder into the wet ingredients.
Stir until just combined, being careful not to overmix.
Fold in half of the chocolate chips.
Pour batter into prepared pan and spread evenly.
Top with remaining chocolate chips.
Bake for 35-40 minutes, until a toothpick inserted in center comes out with a few moist crumbs.
Cool completely in pan before slicing.