Preheat the oven to 350°F. Grease an 8x8-inch baking dish with nonstick spray and line with parchment paper, leaving several inches hanging over the sides for easy removal.
In a small bowl, stir together the flaxseed and 3 tablespoons water. Let sit for at least 5 minutes.
Melt the vegan butter in a microwave safe bowl. Beat in the coconut sugar until smooth and the sugar is mostly dissolved.
Remove the bowl from the heat. Whisk in the milk, flax egg and vanilla.
Add the flour, cocoa powder, baking powder, and salt. Mix with a spatula or wooden spoon until combined. Fold in the chocolate chips (and nuts if using).
Tranfer the batter into the preapared baking dish and spread into an even layer with a spatula. Bake for 25-30 minutes, or until a toothpick in the center comes out mostly clean. Remove from the oven and cool for at least 10 minutes in the pan, then use the parchment paper to lift the brownies out of the pan to a cooling rack. Cool fully before slicing into squares.