Chop 100 g chocolate into small pieces and cut 340 g chocolate cake into bite-size cubes.

Whisk 25 g cocoa powder and 16 g custard powder with 1 cup (240 ml) milk until smooth.

Heat the remaining 1 cup (240 ml) milk with 50 g sugar and 2 tsp instant coffee until simmering.

Pour the cocoa-custard slurry into the simmering coffee milk while stirring continuously.

Cook the custard over low heat, stirring constantly, until thickened enough to coat the spoon.

Remove the thick custard from the heat and stir in the chopped 100 g chocolate until melted.

Cool the mocha custard to room temperature, then chill it until completely cold.

Layer half of the chocolate cake cubes in the trifle dish and cover with half of the cooled mocha custard.

Spread half of the whipped cream over the custard, then repeat the cake, custard, and whipped cream layers.

Top the trifle with 50 g marshmallows and toast them lightly with a kitchen torch before serving.