Preheat oven to 180 degrees celsius. Grease and line an 8" x 12" baking tray.
In a bowl, mix together the ground flaxseed and 150ml of water. Set aside for at least 5 minutes to create a flax egg.
Sift flour and cocoa powder into a large bowl, add sugar, chopped chocolate, and half of the chopped hazelnuts then mix well.
Create a well in the middle of your flour mixture and pour in the melted butter, flax egg, almond milk, and vanilla extract. Combine everything very well until you have a chocolate batter.
Pour batter into prepared tin and then bake in the oven for 40-45 minutes - or until the top is just set and the middle no longer jiggles.
Remove from the oven and cook in the tin completely. Cut the slab into brownie squares and enjoy!