Preheat the oven to 200°C and grease a 28cm tart case with butter.
Pulse the butter and flour in a food processor until the mixture looks like breadcrumbs.
Mix the breadcrumb mixture with icing sugar and vanilla seeds in a bowl.
Add one beaten egg and knead by hand into a dough, then chill for 30 minutes.
Roll the pastry to 35cm diameter, line the tin, add baking beans, and bake for 10 minutes.
Remove beans, prick with a fork, and bake for 5 minutes until golden-brown.
Reduce the oven temperature to 130°C.
Melt the dark chocolate in a heatproof bowl over a pan of simmering water.
Boil the milk and cream in a pan and pour over the melted chocolate.
Add two beaten eggs to the chocolate mixture and stir until smooth.
Pour the filling into the crust and bake for 25 minutes until set.
Serve slices of cooled tart with a spoonful of clotted cream.