Make the spiced custard: In a bowl, mix the vegan custard with pumpkin purée, cinnamon, ginger, and nutmeg until smooth and well combined. Set aside.
Prepare the sponge layer: Cut the vegan cake into small cubes. Split the cake between 4 short tumblers or ramekins.
Make the syrup: Combine the maple syrup and rum in a small jug. Drizzle over the cake cubes to soak slightly.
Crumble the Biscoff biscuits onto the cake, reserving a little for the top.
Layer the trifle: Spoon the blueberry jam over the cake and biscuit layer. Add a generous layer of the spiced pumpkin custard. Sprinkle with chopped nuts and crystallised ginger if using. Reserve some of the nuts and ginger to garnish.
In a large bowl, whip the vegan cream and vanilla, until soft peaks form. Spoon over the top of the trifle.
Garnish with the reserved crushed Biscoff, nuts, and ginger.
