Mix flour and salt in a large bowl. Fill a measuring cup with ice water (you will add it gradually, up to 1 cup). Set aside.

Cut in the fats. Grate frozen butter directly into the bowl (frozen grates cleanly and stays cold longer). Add chilled shortening. Use a pastry blender or fork to work fats into the flour until it looks like coarse crumbs.

Add ice water a spoonful at a time, mixing until the dough just comes together. You may not need the full amount. Do not overwork it.

Gather into a ball, flatten into a thick disc, and wrap in plastic. Refrigerate for at least 2 hours, or overnight. The fats must be cold when the dough hits the oven. The dough keeps in the fridge for up to 3 days, so you can make it ahead. Let it sit out for 10 to 15 minutes before rolling if it feels too firm.

Roll out on a lightly floured surface. Roll from the centre outward into a thin sheet. You want the pastry thinner than you would for a full pie, so the shells are delicate, not thick.

Cut circles slightly larger than the cups of a standard 12-cup muffin tin. Press each circle gently into a cup. Gather the scraps, re-roll, and keep cutting until you have used all the dough.

Chill the shells for 15 minutes before filling, so they hold their shape in the oven.

Preheat your oven to 400°F. Place your filled tart shells on a baking sheet (this catches any overflow and makes them easier to move).

Make the filling. Whisk the melted butter and brown sugar together until combined. Add the vanilla, eggs, and cream and whisk until smooth. The filling will be thin, and that is what you want.

Fill the shells about two thirds of the way. They puff up in the oven and you do not want them to overflow.

Bake for 15 minutes, until the filling is set around the edges but still has a slight jiggle in the centre. It will firm up as it cools.

Cool completely in the pan before removing. The filling needs time to set. I know it is hard to wait.