Incorporate the margarine into the flour (you can do this by hand, in a bowl or a food processor), add a pinch of salt, and 4 Tbsp of ice-cold water (one at a time).
Mix well until the dough is formed. Let it rest in the fridge for 1 hour.
Melt the chocolate over a steamer or in the microwave and let it come to room temperature.
Put the chickpea liquid into a separate bowl, add the cream of tartar and beat with a hand mixer.
When it foams up, add the sugar a little at a time. Beat until stiff peaks.
Add the vanilla and orange zest.
Fold in the melted chocolate and crushed walnuts.
Grease and line your baking tray.
Split the dough into two equal parts.
Roll the dough in the shape of your tray.
Place one layer of dough on the bottom of your pan, top with your chocolate-walnut mixture, and cover with the second layer of dough.
Bake at 180 degrees C for 40 minutes or until a toothpick comes out clean.