Preheat the oven to 200 degrees Celcius or 392 degrees Fahrenheit.
Mix the flax seeds with the water and let it stand for 5 minutes.
Briefly whip the chickpea water with a pinch of salt, then add the sugar and whip everything for 5 minutes until a firm snow has formed.
Mix the flaxseed mixture, oil, vegan milk and agave syrup in a bowl with a whisk, then add all the dry ingredients. Finally fold in the whipped snow.
Pour the mixture onto a baking tray lined with baking paper. Bake the gingerbread sheet cake in the oven for 20 minutes.
If you like, you can cut the cake into large pieces and cover them with dark chocolate or garnish them with icing sugar.
If you store the bars in an airtight container in a dark room without draught, they will keep for 1 to 2 weeks.