Thaw frozen puff pastry according to package directions.

Unroll the puff pastry, then spread 2 tbsp butter thinly and evenly across one side of the sheet.

Roll it up tightly with the butter on the inside (see video), then wrap it tightly in plastic wrap. Let it sit in the fridge to firm up — you can also make this ahead of time.

In a microwaveable bowl, add milk and heavy cream. Microwave for ~20 seconds or until the mixture is warm.

Add sugar, 1 whole egg, 2 egg yolks, vanilla extract, and salt. Whisk until well combined.

Preheat oven to 400F (convection on).

Take the puff pastry out of the fridge and remove the plastic wrap. Slice into 12 even-sized discs.

Arrange 12 egg tart tins on a baking sheet tray.

Place each disc into 2.5 or 3 inch egg tart tins, then use both thumbs to evenly press into the center while rotating the disc in a circular motion. Gradually move your thumbs outwards to expand the pastry to the top of the tin, ideally a little over the top.

Repeat with the rest of the puff pastry discs. Give the custard a quick whisk, then fill each puff pastry shell only 80% of the way to the top, leaving some crust exposed. It's better to slightly underfill than overfill, as overfilling will cause the custard to spill over to the crust, making it soft and not crispy!

Place the tray of egg tarts into the oven, and bake for 12-14 minutes in a small countertop oven or 16-18 minutes in a traditional oven, or until the tops of the custard and crust become beautifully golden brown.