Place all the ingredients in a bowl, rub together with your fingers to create a crumbly mixture.
Using the whisk on a mixer, beat the margarine, sugar, vanilla and salt. Mix the chia seeds with the water, stir in with the apple sauce. Mix the flour and baking powder, stir into the batter. Transfer the batter to the prepared tin, smooth down. Spread the hazelnut crunch on top, sprinkle with salt.
Approx. 1 hr. 20 mins. in the centre of an oven preheated to 170°C. Remove from the oven, allow to cool slightly, remove from the tin and leave to cool completely on a rack.