Preheat oven to 180°C Fan and line a 8×8 inch square baking tin with grease proof paper.
In a small bowl, add in the flaxseeds and water, stir to combine then set aside for 10 minutes. It will thicken and turn jelly-like 'egg' consistency.
Fill a small saucepan 1/4 full with water and place on the hob over medium heat. Add 3/4 of the dairy-free chocolate into a heat proof bowl. Place the bowl over the pan, creating a bain-marie. Allow the chocolate to melt, stirring with a heat proof spatula. Remove from the heat and allow to cool for a few minutes. You can also melt the chocolate in the microwave.
Add the dairy-free butter / margarine into a saucepan and melt. Pour in the sugar and whisk with a hand whisk to combine. Pour the butter and sugar mixture into a medium sized mixing bowl.
Add the flax ‘egg’ into the butter and sugar, along with the melted chocolate and vanilla. Whisk to combine.
Sift in the cocoa powder, baking powder and gluten-free flour. Fold the batter together until combined.
Pour the brownie batter into the lined tin, top with the remaining 1/4 chopped chocolate and bake for 25-28 minutes. Once baked, remove from the oven and allow to cool at room temperature for at least 3 hours.
Once cool, transfer into the fridge to set the chocolate fully. You can enjoy it warm in a bowl with some ice-cream or allow to set up in the fridge for a firmer, fudgey brownie.