Preheat oven to 400°F/200°C. White chocolate needs to firm back up after melting, so tackle the cream first: whip the heavy cream to stiff peaks and set aside.
Melt the white chocolate over a double boiler, stirring gently to prevent scorching. Cool slightly - it's good if it's pourable.
Beat the cream cheese until fluffy. Mix in the melted chocolate and beat until fully combined. Cool the mixture in the fridge for 10 minutes.
Fold in the whipped cream by hand with a spatula, gently, to keep the mousse airy. Return to the fridge while you roast the plums.
Halve and pit the plums. Arrange them on shallow baking tray, drizzle with honey.
Roast in the oven for 30 minutes, turning halfway, until the plums release a thick, syrupy juice that turns glossy.
Cool the roasted plums to room temperature.
Layer into glasses: first plums, then the chocolate mousse; repeat to the top.
Chill in the fridge for 1 hour before serving. Top with plum halves and crushed nuts or cookies before you serve them. Enjoy :)