Preheat oven to 170C. Combine muscatels and 50ml Marsala in a small saucepan, bring to the simmer over medium heat, remove from heat and cool to room temperature.
Meanwhile, beat 200gm brown sugar and butter until pale and creamy, add eggs and chestnut purée and beat to combine. Sift over flour, 50gm cocoa and baking powder, mix to combine, then stir in milk, chocolate, muscatels and muscatel liquid. Divide among 6 lightly buttered 2-cup-capacity ovenproof moulds and set aside.
Combine remaining brown sugar and remaining cocoa in a heatproof bowl, gradually add 350ml boiling water and stir until smooth and combined, then stir in remaining Marsala. Pour over pudding batter, dividing equally among moulds, then bake until risen and a skewer withdraws clean (15-20 minutes).
Meanwhile, for chestnut cream, whisk ingredients until soft peaks form, then refrigerate until required. Serve with hot chocolate-chestnut puddings.