Place egg yolks, sugar, cornflour and vanilla in a bowl and whisk until smooth. Add cream and 125 ml water and whisk again. Pour into a saucepan and place on medium heat. Stir until mix is thick, about 5 minutes. Remove from heat, add chocolate and stir until melted. Leave to cool.
Place the sheets of puff pastry on top of each other and roll up. Cut into 1 cm wide slices and roll out rounds 10 cm in diameter. Lightly grease a non-stick muffin pan and push each round into the holes. Place in freezer for 10 minutes. Remove, and divide mix evenly between the tart shells.
Place in oven and bake for 25 minutes or until pastry is golden. Cool in tin for 10 minutes. Dust with icing sugar and serve with cream fraiche.Makes 18.