While the dough is chilling in the fridge, cut the mirabelle fruits lengthways and remove the small pit in the middle. Do this to all the fruit.
Once all the fruit is halved add them to a bowl together with the icing sugar. Making sure to cover all the pieces with the sugar.
Preheat the oven to 180°C or 350°F. Spray the tartlet tins with some baking spray and set aside.
Divide the chilled dough into 6 equal portions.
Open each piece of dough into a circle, slightly bigger than the tartlet tin, between two sheets of cooking paper. Do not add any additional flour.* Place the dough over the tartlet tin and gentle press it in. Remove any excess dough from the edges. Poke several fork holes at the bottom.
Once all the tart bases are done, divide the crushed pistachios between each tin and gently layer it at the bottom. Cover the pistachio layer with the sugared mirabelle halves. Place each half vertically resting on each other. Fill it up completely until no space is left.
Divide the butter into 6 equal cubes and place at the top of the tart in the middle. Bake the tartlets for about 15-20 minutes at 180°C.
As soon as you take them out of the oven, sprinkle some crushed pistachios on top and let them rest for a few minutes. These tartlets can be served either slightly warm or cold.