Stir together the ground flaxseed and warm water in a small bowl. Let sit for 10 minutes until thickened and gel-like. This is your egg replacement.

Preheat oven to 350°F. Line an 8x8-inch pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly grease any exposed pan surface.

In a medium saucepan over low heat, combine Nutella, butter, and chocolate chips. Stir constantly until completely smooth and glossy, about 4-5 minutes. Remove from heat and let cool 2 minutes so you don't cook the flax egg.

Whisk sugar and vanilla into the warm chocolate mixture until dissolved. Add the thickened flax egg and whisk vigorously for 30 seconds until fully incorporated. The batter should look thick and shiny.

Sift flour, cocoa powder, and salt directly into the bowl. Fold with a rubber spatula until just combined and no dry streaks remain. Do not overmix - a few lumps are fine.

Scrape batter into prepared pan and smooth the top. Bake 22-26 minutes until the edges look set and a toothpick inserted 2 inches from the edge comes out with moist crumbs. The center should still look slightly underdone - it will set as it cools.

Cool completely in the pan on a wire rack, at least 1 hour. Use parchment overhang to lift out, then cut into 16 squares with a sharp knife. Wipe the knife clean between cuts for neat edges.