Heat the oven to 180C/160C fan/gas 4. Line the base of a deep 23cm springform cake tin with baking parchment. Whisk the oil, milk, vanilla and egg together in a jug. Combine the flour, bicarb, cocoa and sugar in a bowl using an electric whisk. Add the wet ingredients to the dry, and whisk until smooth and just combined. Pour into the tin and bake for 18-20 mins until risen, firm and a skewer inserted into the middle comes out clean. Cool in the tin for 10 mins, then turn out onto a wire rack, top-side down – this will help flatten it if it’s slightly domed.

Meanwhile, make the custard. Put all the chocolate in a large heatproof bowl. Warm the 130ml milk and 350ml of the cream in a medium pan over a low heat until steaming. Combine the cornflour and the 1 tbsp milk in a medium bowl until smooth, then whisk in the eggs and sugar. Slowly pour in the warm cream mixture, whisking all the time. Wipe out the pan and return the custard to it. Warm gently, stirring continuously until the custard has thickened and reached 76C. Pour this over the chocolate and leave for a few minutes to melt, then stir until smooth. Stir in the liqueur, if using.

Pour the custard into a bowl and cover with a disc of parchment. Cool. Once completely cool, whisk the rest of the cream to soft peaks and fold this into the custard.

Clean the cake tin and line with parchment. If the sponge is domed, trim the top with a serrated knife. Put the sponge in the tin, bottom-side up. Spoon over the custard, smooth the surface and chill for 6 hrs to set.

For the ganache, put the chocolate in a heatproof bowl. Warm the cream and glucose in a small pan over a low heat until the glucose has melted and the cream is steaming. Pour this over the chocolate. Leave for a few minutes, then stir until smooth. Cool at room temp until thick but still liquid. Pour this over the custard. Level the surface and chill for 2 hrs to set. Decorate with the stars and truffles, sprayed with gold spray.