I got this recipe from Vegan Baking and it was a bit fiddly but the brownies turned out really well.
They go a bit hard once cooled but get soft again if you reheat them on the stove or microwave.
Chocolate Walnut Brownies
3 Tablespoons water
2 Tablespoons flax meal
1 ¾ cups all-purpose GF flour
¼ teaspoon baking soda
7 Tablespoons cocoa powder
4 ounces semi-sweet chocolate, chopped into small pieces
1 teaspoon instant espresso powder (optional)
¾ teaspoon salt
¼ cup boiling water
1 ½ cups sugar
6 Tablespoons (80 grams) Regular Vegan Butter or non-hydrogenated stick margarine, melted (not tub margarine)
1 ½ teaspoons vanilla extract
½ cup walnuts, chopped (optional)
In a small mixing bowl whisk together the water and flax meal. Let it sit for about 10 minutes so the mixture gets goopy. Place your oven rack on it's lowest position and preheat your oven to 350F. Line an 8 x 8 inch baking dish with parchment paper allowing excess parchment paper on opposite sides so you can pull the brownies out of the baking dish later.
In a medium mixing bowl whisk together the all-purpose flour and baking soda. Set aside.
In another medium mixing bowl add the cocoa powder, semi-sweet chocolate, espresso powder and salt. Add the boiling water and using a spoon, mix in the ingredients into a paste, making sure that all of the chocolate pieces are melted. Add the sugar, Vegan Butter, vanilla extract, flax meal mixture into the chocolate mixture and mix with an electric mixer until smooth. Stir in the walnuts if you're using them. Using a spoon, mix in the flour until well combined. The mixture will become extremely thick and you may need to use your hands to mix. Refrain from mixing this dough with an electric mixer at this point; this will overwork the dough and activate too much gluten, negatively affecting the texture of the brownies.
Transfer the batter to the baking dish. You may need to use your fingers or a spatula to press the thick dough into place. Bake for 35 minutes on your oven's lowest rack. Transfer the baking dish to a wire cooling rack and allow it to cool for about an hour.
Transfer the brownies to the wire rack by lifting them out of the baking dish by taking hold of the exposed parchment paper on each side. Allow them to cool completely before slicing into squares.