Preheat your oven to 350°, and line the bottom of four ten-centimeter pie tins with parchment paper and set aside.
In the bowl of a stand mixer cream the butter, sugar, grapefruit zest, grapefruit juice and vanilla until well combined. Gradually add the flour and salt and mix until the dough starts to come together.
Turn the dough out onto a cutting board and form it into four small disks. Wrap in plastic wrap and chill in the fridge for 20 minutes.
Once the dough is chilled, roll out each disk in to six millimeter thick circles. Place dough in prepared pie tins, trim the edges and make a fluted edge with your fingers. Using a fork, lightly poke holes in the dough. Bake the crusts in the oven for 12-16 minutes, or until the edges are a light golden brown. Set the pie tins aside and let them cool.
Using a double boiler (a heat proof bowl over a pot of simmering water) whisk together yolks, sugar, orange juice, zest and lemon juice. Continue to whisk until completely combined.
Carefully remove the bowl from the pot (the bowl will be hot!) and immediately whisk in the cubbed butter. Strain the curd into a fine mesh strainer and let sit for about 20 minutes. Sprinkle gelatin powder over 1/4 cup hot water and mix until dissolved. Pour gelatin mixture in to the curd and mix well. Immediately pour curd into prepared tart shells and place them in the refrigerator to set, about 20 minutes.
Once the curd has set in the tarts, top with whipped cream, citrus segments, mint and chopped pistachios. Enjoy!