Prepare the shortcrust pastry following the recipe link above.
While the pastry rests in the fridge, let's prepare the filling. Simmer chestnuts in rice milk until the latter is almost absorbed and chestnuts are tender.
Remove the chestnuts from heat and add the chocolate, chopped into chunks. Stir until the chocolate is melted.
Season the chestnuts and chocolate filling with ginger and nutmeg, add the starch using a sieve and finally add plant cream. Blend with an immersion blender to get a smooth cream.
Take 3/4 of the pastry and roll it out to half a centimeter thick round sheet. Line a 9 inch springform pan with the pastry, then pour in the chestnut and chocolate cream. Use the remaining pastry to create a decorative upper crust.
Bake at 375°F for 40 minutes, then check for doneness with a toothpick: when the tart is done the filling should be dry.
To fully appreciate the full-bodied flavor of chocolate chestnut filling let the pie cool down before serving.