Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line a 20cm square tin with greaseproof paper.
Place the flaxseeds in a small food processor and blitz until very fine and powdery. Mix with 5 tbsp cold water and set aside for 5 minutes.
In a large mixing bowl, combine the brown sugar, cacao, ground almonds and baking powder. Mix together the maple syrup, oil and almond milk in a jug with 50g tahini. Gradually add the wet ingredients to the dry ones until it forms a smooth mixture, then stir through the flaxseed mixture.
Transfer this to the lined baking tin. Drizzle over the remaining tahini paste. Bake for 40-50 minutes, until a skewer comes out mostly clean when inserted. Leave to cool for around 10 minutes, then remove from the pan and leave to cool slightly before serving.