Heat the heavy cream, milk, cinnamon stick, and lemon zest in a saucepan. Bring to a boil for a moment, then remove from heat, cover, and let steep for 20 minutes.

Strain the mixture over the cornstarch and whisk until the cornstarch is dissolved. Return to the heat and cook, stirring constantly, until thickened.

Make a syrup by boiling the sugar and water to 240°F (115°C), or boil for 4 minutes after it starts bubbling.

Mix the custard with the syrup, stirring well.

Strain the egg yolks, and when the custard has cooled to room temperature, add the yolks and mix until fully combined.

Grease round molds with butter. Roll out the puff pastry (gluten-free or regular), cut circles slightly larger than the molds, and press them into the molds. Fill with the custard.

Bake in a preheated oven at 480°F (250°C) for 10-12 minutes, or until the tops are golden brown. Use both top and bottom heat if possible.