Grease or line a 20cm square baking tin with baking paper. If using the baking paper, leave it to stick out of the tin on all sides. That way it's easier to pull the baked brownie out of the tin.
In a small bowl, combine the ground flaxseed and water and set aside for 10 minutes.
Whisk the melted coconut oil and sugar until combined. Add vanilla extract, flaxseed and melted chocolate and whisk until smooth.
In a separate bowl, combine the flour, baking powder, cocoa powder and salt. Sift into a bowl with wet ingredients and using a wooden spoon or silicone spatula, stir until combined.
Finally, add the plant-based milk and stir one more time until smooth (but thick) mixture is obtained.
Transfer into the prepared tin and smooth out the top with the back of a wooden spoon.
Bake for 25 minutes or until a toothpick inserted into the centre comes out clean.
Cool for at least 45 minutes before decorating.
When ready to decorate, melt the marshmallows in the microwave for 20-30 seconds, allow to cool for 5 seconds.
Put your fingers into melted marshmallows, stick them together then pull them apart. As you're pulling your fingers apart, melted marshmallows will create the cobweb look.
Attach the melted marshmallows on the edges of the brownie until whole surface is covered.