I love this recipe because is so versatile. You can use this cremeux both for filling (as in the tiramisu tartellette) and for decorating (as in the photo).
Ingredients
64g Dark chocolate 70%
2 Egg yolks
14g Caster sugar
120g Whole milk
30g Heavy cream (35% fat)
40g Unsweetened coffee
Process
In a bowl, lightly whisk sugar and egg yolks.
In a small saucepan, heat the milk and the heavy cream until they start to simmer, then slowly pour everything into the egg yolks while whisking.
Stir the mixture over low heat until the custard thickens (it should reach around 80 °C).
Remove the custard from heat and pour into chocolate while stirring with a spatula until smooth and glossy. You can use a strainer to pour to avoid having eventual lumps.
Add the coffee and stir the mixture again. I'm using here 40g of espresso for a strong flavour but you can adjust to taste. If you use more coffee you might need to add some gelatine (e.g. 1g of dehydrated gelatine). In general I would recommend to use a strong espresso but not a lot.
Refrigerate overnight.