Preheat the oven to 350 degrees F. Lightly grease an 8x8 inch baking dish with cooking spray.
Make your flax egg by combining the flax seed and water in a small bowl. Let set for 5 minutes before using.
Mix together the dry ingredients in a medium mixing bowl: sugar, cocoa powder, flour, baking powder, and salt. Pour in the wet ingredients: peanut butter, canola oil, non-dairy milk, flax eggs. Mix until a thick but spreadable batter forms.
Spread the brownie batter into the greased 8x8 inch pan.
Mix together the peanut butter swirl ingredients, adding additional milk as needed to make drizzle-able. Drizzle over the brownies in lines. Drag a knife through the lines to make swirls.
Bake in the oven for 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Let brownies cool before slicing. Enjoy!
I recommend using natural peanut butter because that's the only kind I used during recipe testing. Regular peanut butter will likely yield similar results but hasn't been tested.An 8x8 inch baking pan makes thicker brownies that take 35-40 minutes to bake through. If you use a 9x13 pan you'll want to decrease the baking time to 30-35 minutes. Always make sure to test the center with a toothpick to confirm the brownies are baked through.