Beat the egg, sugar, and salt in a bowl until the mixture becomes frothy.
Add the flour all at once and mix with a spatula, then work the dough with your hands until it becomes crumbly.
Add the butter and knead to obtain a homogeneous dough and shape into a ball.
Wrap the dough in plastic wrap and refrigerate for 1 hour.

Preheat the oven to 375°F (190°C)
Roll out the dough and place it in a 11″ (27.5cm) buttered tart mold.
Spread the almond powder over the bottom of the tart.
Arrange the plums, cut into quarters, tightly packed.

In a bowl, beat the eggs with the cream, milk, and sugar.
Pour the mixture over the fruit.
Bake for 45 minutes to 1 hour, or until the tart is golden brown and the topping is set.