To make the chocolate pâte sucrée, beat the butter in a mixing bowl until soft and smooth. Add the icing sugar and beat until smooth. Gradually incorporate the eggs and mix until fully emulsified. Mix in the flour, cocoa powder and salt until smooth. Turn the dough out onto a work surface, shape it into a block and wrap in cling film. Leave to rest in the fridge for 2–3 hours.
Meanwhile, to make the almond cream, beat together the butter and sugar in a mixing bowl until smooth. Gradually beat in the eggs until combined. Fold in the ground almonds and flour and mix until smooth.
To make the orange marmalade, use a peeler to remove the orange peel in long strips, then thinly slice it and place in a muslin bag. Seal. Roughly chop the oranges and lemon, put in a saucepan with the sugars, the muslin bag and 250ml/9fl oz of water. Simmer over a low heat, uncovered, for 2 hours, or until the fruit is tender.
Remove the muslin bag and set aside to drain. Line a colander with layers of muslin and set over a bowl. Strain the contents of the pan through the muslin and leave to drain for 30 minutes. Squeeze out the remaining liquid by twisting the cloth. Discard the pulp.
Return the strained liquid and the peel from the muslin bag to a clean saucepan, bring to the boil and boil until it reaches 104°C, stirring continuously. To test the marmalade is setting, spoon a small amount onto a cold plate and leave to cool. If it sets, remove the pan from heat and spoon the marmalade into a sterilized jar. Leave to cool, then seal.
To make the orange syrup, put the sugar, lemon peel and vanilla in a saucepan with 225ml/8fl oz of water and bring to the boil. Cook for 2–3 minutes. Remove from the heat and allow to cool. Once cool, add the liqueur.
To make the chocolate mousse, put the milk and 75ml/2½fl oz of whipping cream in a saucepan and bring to the boil. Meanwhile, whisk together the egg yolks and sugar in a large mixing bowl for 2–3 minutes, or until light and pale. Pour half the boiled milk and cream into the egg mixture and mix well. Pour this back into the pan, mix and cook over a low heat, stirring continuously, until it is thick enough to coat the back of a spoon.
Put the chopped chocolate into a large bowl. Take the egg and cream mixture off the heat and pass through a fine sieve onto the chocolate. Use a spatula to mix until the chocolate has fully melted and the mixture has emulsified. Allow to cool.
Whisk the remaining 200ml/7fl oz of whipping cream in a large mixing bowl until soft peaks form. Carefully fold the whipped cream into the chocolate mixture and use immediately.
To make the chocolate glaçage, soak the gelatine in a bowl of iced water for a few minutes, until soft. Squeeze to remove any excess water. Put the sugar in a saucepan with 225ml/8fl oz of water, bring to the boil and simmer on a low heat for 2–3 minutes. Add the cocoa powder and cream. Bring back to the boil and simmer for 4–5 minutes. Take the pan off the heat, add the soaked gelatine and stir until dissolved. Strain and allow to cool.
To assemble, fill the mince pie tins with the chocolate mousse. Flatten the tops with a small stepped palette knife, cover and place in the freezer to set for at least 2 hours.
Roll out the pastry to 3mm thickness and line the tartlet cases. Cover and rest in the fridge for 1 hour.
Preheat the oven to 200C/180C Fan/Gas 6. Remove the tarts from the fridge and pipe 10g/⅓oz of marmalade into the base of each one. Pipe a bulb of almond cream on the top, cover and leave to rest in the fridge for 30 minutes.
Bake the tarts for 15 minutes. Allow to cool, then remove them from their cases. If necessary, level off the tops using a serrated knife.
Take the set mousses out of the mince pie moulds and place on a glazing rack. Glaze with the glaçage and place one on top of each tartlet. To serve, decorate with dried orange zest, a chocolate stick and gold leaf.