Preheat oven to 180 degrees C. Grease a tart tin with vegetable oil.
Melt the coconut oil in a small saucepan. In a food processor, crush the biscuits then add the coconut oil and almond milk. Mix until it comes together. Press into the tart tin base and up the sides. Bake 15 minutes and cool.
Meanwhile, preparing the filling. Place the agar in a small bowl and pour over the boiling water and stir until dissolved.
In a small saucepan, combine sugar, cocoa powder and cornflour. Over medium heat, stir in half the almond milk until smooth. Add the rest of the almond milk while continuing to whisk. Stir until thickened and simmering, about 10 minutes.
Remove from heat and add diced chocolate, cardamom, zest and dissolved agar. Stir until smooth and chocolate has melted.
Pour into the tart shell and let set 3 hours in the refrigerator.