Preheat the oven to 400°F (200°C).

Let the pre-made tart shells sit at room temperature for 10 minutes.

Bake the shells on the lower rack at 400°F (200°C) for 13-15 minutes (do not bake longer). Remove and let cool.

Mix the whole egg and egg yolks in a bowl using a whisk until combined. Add powdered sugar and mix until just combined.

Add heavy cream, milk, vanilla extract, and salt. Stir until everything is well combined, then strain the mixture.

Pour the mixture into the pre-baked tart shells, filling them almost to the top (be careful not to spill, as it can cause the crust to stick to the mold and look unattractive).

Bake on the middle rack at 400°F (200°C) for 28-30 minutes. Check the tarts as needed, as ovens may vary.

Let the tarts cool slightly before serving warm.

If not consumed immediately, store in the refrigerator and reheat before serving.