Make the flax egg by mixing the flaxseed and hot water in a bowl. When it's ready to use, it looks gloopy and shiny. Set aside.
Preheat the oven to 180°C (gas mark 4.5). Grease your baking tin or line with baking paper. I used a baking tin in size 11 x 7 inch but anything similar is fine.
Sift the flour, baking powder, cocoa powder and salt into a mixing bowl and add in the sugar. Mix well.
Melt the margarine in a microwave in a bowl and mix in the flax egg, stir well. Pour this wet mixture into the dry mixture and mix until well combined. The combined mixture should be firm, almost like cookie dough.
Put the mixture into the prepared tin and press evenly with a spoon or a folk. The thickness should be somewhere between 1 and 2cm.
Sprinkle the top with some sugar and make lines with a knife for easy cut for later. Prick the top with a folk. Bake for about 20-25 minutes or until the edges are looking slightly crunchy. Every oven is different so please check your cake when you think it's time☺
Let it cool completely before taking out of the tin. You should be able to cut the cake easily with your hand or a knife as you made the cut lines already before making!
Store leftovers in an airtight container at room temperature up to 3 days. You could keep the cake in the fridge but they might go a little dry.