In a medium-sized bowl, combine the egg yolks and the sugar, beat lightly and set to one side.
In a medium-sized pot, combine the milk and the cream and place it over a low heat.
Lengthwise, split the vanilla pod and scrape out the seeds, adding both seeds and pod to the pot. Bring to the boil. While stirring, add a little of the hot liquid to the egg mixture. Return to the pot and stir energetically until the sauce thickens. Remove the vanilla pod and leave to cool.
Grease a medium-sized pastry mould with butter and dust it with flour. Carefully place the pastry dough in the mould, folding the edges, pressing the dough into the corners of the mould with your fingers.
Bake in a pre-heated oven at 350°F (175°C) for 20 minutes until golden and firm to the touch. Leave to cool.
Slice each plum in 2 and remove the stone. Then, cut each half into 3 pieces. Place in a baking dish and bake at 250°F (125°C) for 20 minutes until semi-dried.
Pre-heat the broiler in your oven. Cover the pastry with the vanilla custard, and place the plums in an attractive pattern on top. Garnish with marshmallows, and sprinkle with cognac or brandy. Place under the broiler and bake for 2 minutes or until the marshmallows begin to melt.