Preheat the oven to 180c (fan). Grease and line a 32 x 24 cm sheet tin with baking parchment. Set aside.
Measure the psyllium husks or flax seeds into a spice grinder, high speed blender or mortar and pestle. Grind/blend until the husks/seeds are roughly ground. Transfer the ground seeds to a small bowl and add the water. Whisk well and leave to hydrate.
In a medium bowl, combine the plant milk, lemon juice, vanilla, vegetable oil and caster sugar. Whisk lightly to combine. Add the hydrated, ground psyllium or flax. Either transfer to the cup of a high-speed blender or use an immersion blender to completely blend the mixture. The blended mixture should be around the same texture as mayonnaise - thick, creamy, smooth and pale. Set aside.
In a medium bowl, sieve together the flour, baking powder, bicarbonate of soda and salt. Whisk together then add to the blended wet ingredients. Fold in until just combined, making sure no lumps of flour remain. Finally add the chocolate chips, reserving a small handful for on the top of the cake.
Transfer the batter to the prepared sheet tin and level the top with a spatula. Top with the reserved chocolate chips before placing in the oven. Bake for 40 minutes, rotating the sheet tin half-way through.
Once baked, a toothpick to the centre of the cake (avoiding chocolate chips) should come out clean and the top should be golden brown. Place on a wire cooling rack to cool fully (around an hour) and do not remove the cake from the tin. Once cooled fully make the ganache.