Years ago, I saw a recipe for caramel blood oranges from Claudia Fleming. Thankfully, the idea lodged itself in my brain: it serves as the inspiration of these tartlets. I opted for ruby-red grapefruit since they're more readily available than blood oranges. The combination of the tart grapefruit with the rich, salty caramel sauce is pretty unforgettable. All components (the tart shell, the filling, and the caramel grapefruit sauce) can be prepped in advance, making these a nice option for entertaining.
To toast hazelnuts: Place hazelnuts in single layer on rimmed cookie sheet. Toast for 8 to 10 minutes in a 350 degree F oven, shaking the pan once or twice to ensure even browning. Once cooled, put toasted hazelnuts in the freezer for 15 minutes or longer.
To grind hazelnuts: Combine frozen hazelnuts with powdered sugar in bowl of food processor. Pulse -- stopping every few seconds -- until the hazelnuts are uniformly and finely ground, about 15 to 20 pulses.
Preheat oven to 350 degrees F.
Prepare crust: Put flour, hazelnut - powdered sugar mixture, cocoa and salt in the bowl of a food processor and pulse a few times to combine. Add the butter and pulse until coarsely cut in, about the size of peas -- about 5 to 7 times. Stir the yolk to break it up, pour over the dry ingredients, and pulse (about 5 seconds each time) until the dough starts to clump and come together. If some of the dough is still dry and crumbly, that’s okay, since it’s a press-in crust. Divide the dough evenly among six 4-inch tart pans (or one 9-inch tart pan), pressing the dough evenly over the bottom and up the sides of the pans. Prick the dough all over with a fork, and freeze for 30 minutes or longer.
Bake crust: Bake for about 25 minutes; the dough should be firm to the touch. Keep a close eye on the crust to make sure it doesn’t burn -- you can always lay a sheet of foil across the top if it’s browning too quickly. About half-way through the baking time, check the dough to see if it has puffed up; if so, press it down with the back of a spoon so you’ll have a deep enough shell for the filling. Remove from the oven, and cool completely.
Prepare filling: In a mixer fitted with the whisk attachment, combine mascarpone, Greek yogurt, powdered sugar, grapefruit zest, and grapefruit juice. Whisk until well combined. Chill until ready to assemble the tarts. (I like to place the filling in the freezer for about 5 to 10 minutes before assembling the tartlets so it's nice and cold.)
Prepare caramel grapefruit sauce: Slice off the top and bottom of grapefruit, then with a curving cut, remove the peel, working in sections. Now the flesh should be completely exposed. Cut away the segments of fruit from the membranes that connect them.
Place ½ cup water in a small saucepan, and then add the sugar and corn syrup. Cook over medium-high heat until the sugar dissolves. Cook the caramel, swirling the pan occasionally, until it turns a deep amber -- about 7 to 10 minutes. Immediately take the pan off the heat.
Add ¼ cup water to the pan -- stand back, the caramel may splatter! Set the pan over low heat and cook, stirring, until the caramel dissolves and is smooth, about 5 more minutes.
Turn off the heat. Add the grapefruit segments and a pinch of fleur de sel and gently stir to combine. Let cool a few minutes before assembling tartlets. (The sauce can be prepared in advance and gently reheated before assembling tartlets.)
Assemble the tartlets: Spread the mascarpone filling into the tartlet shells, smoothing the top. Spoon the caramel grapefruit segments over the top, taking care to not shortchange anyone on the caramel sauce.