Preheat the oven to 180°C and grease a 12-hole muffin tin with butter.
Whisk egg yolks, mustard, and cornflour in a saucepan over medium heat until thickened.
Add vanilla seeds, then gradually whisk in the milk and cream until the mixture is thick and smooth.
Bring the custard to a boil while stirring, remove from heat, stir in thyme, and cool with clingfilm on the surface.
Roll out puff pastry, stack two halves, and roll into a cylinder before cutting into 12 slices.
Roll each pastry slice into a 10cm circle on a floured surface.
Press pastry discs into the muffin tin and fill them with the cooled custard.
Bake for 20 minutes until the pastry is golden-brown and the custard is set.