Once the cake is completely cool, use a large serrated knife to carefully slice it into 3 layers. Spread the plum jam evenly across the two bottom cake layers, then reassemble the cake. Set it aside while you prepare the glaze.
In a medium pan, combine the double cream, dark chocolate, and honey. Cook this over a low-medium heat, stirring occasionally, until the chocolate is fully melted and you have a smooth, pourable mixture. Place the cake on a wire rack set over a parchment-lined baking tray, then pour the glaze over the cake, ensuring it's completely covered. Use a fish slice to carefully transfer the glazed cake to a serving plate and allow the glaze to set.
If you like, dust with a little edible gold powder for decoration. Uniced, this cake will keep for up to 5 days. Once iced, store it in the fridge and enjoy within 3 days; for the best texture, remove it from the fridge and let it come to room temperature before serving.