These delicious little tarts combine the creamy richness of panna cotta with the bright floral qualities of fresh citrus, creating one of the most delicate and pleasing flavor combinations of any dessert. Add in a nutty and nourishing pistachio walnut crust, and you've got a real winner on your hands!
In the bowl of an electric stand mixer fitted with the paddle attachment, beat together the butter, sugar, and vanilla extract at medium speed until smooth. Add the egg yolk and mix at low speed until combined. Add the ground walnuts, ground pistachios, cardamom, and salt and mix until blended. Then add the flour and stir until a crumbly mixture forms.
Pat the mixture into the bottom and sides of 6 different _-inch tart pans with removable bottoms. Press the mixture tightly against the sides and bottoms of each pan to solidify it, creating a uniformly 1/4-inch thick tart crust within each pan. Place the pans in the freezer for 20 minutes.
Pat the mixture into the bottom and sides of 6 different 5-inch tart pans with removable bottoms. Press the mixture tightly against the sides and bottoms of each pan to solidify it, creating a uniformly 1/4-inch thick tart crust within each pan. Place all of the tart pans on one lipped baking sheet, and place the pans in the freezer for 20 minutes.
Preheat the oven to 350 degrees fahrenheit. Poke each tart all over with a fork and then bake them in the oven for 10 minutes.
Remove them and pat down any air bubbles rising in the crust. Brush the interior pf each crust thoroughly with the egg white mixture, making sure to cover all the little holes. Place the pan back in the oven and bake for an additional 10 to 15 minutes, or until the edges of the crust are golden and the egg wash is baked on. Remove from the oven and allow to cool completely on a wire rack. Leave the tarts in the pans for now.
Pour the milk and cream into a small saucepan and sprinkle the gelatin over the top. Let it sit at room temperature for 5 minutes.
Turn the heat on medium low and stir until the gelatin dissolves. Add the sugar and vanilla and stir until the sugar dissolves.
Remove the pot from the heat and place the pot in an ice bath, whisking constantly until the mixture cools to room temperature.
Remove the pot from the ice bath. At this point, make sure you have room in your freezer for all the tart pans to rest flat.
Distribute the panna cotta between the tart pans, filling each about halfway with the panna cotta mixture. Place the pans in the freezer and allow to chill for 15 to 20 minutes, or until the top of the panna cotta layer is set.
While the panna cotta is chilling, you can prepare the citrus gel. In a small saucepan, sprinkle the gelatin over the citrus juice and allow to rest at room temperature for 5 minutes.
Place over low heat and add the sugar and stir *just* until the gelatin and sugar have dissolved, then immeditately remove from heat and place the pot in an ice bath, whisking constantly until the mixture cools to room temperature.
Remove the pot from the ice bath, then remove the panna cotta tarts from the freezer and evenly distribute the citrus liquid between the tarts, leaving a bit of space (about 1/8 inch) at the top of each tart.
Place the tarts flat in the refrigerator and chill until the citrus layer is completely set, about 2 hours.
Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment and set aside.
In a small saucepan, stir together the sugar and the water over high heat. When it begins to boil, stop stirring and place a candy thermometer in the pot.
When the temperature reaches 230 degrees Fahrenheit, begin beating the egg whites at medium speed. Ideally, when the egg whites form soft peaks the mixture will be 240 degrees Fahrenheit. Increase the speed to medium high. At this point, remove the pan from the heat and VERY carefully drizzle it into the egg white mixture while the mixer if beating them, drizzling the syrup in a steady fine stream.
When all the syrup is incorporated, add the vanilla extract and continue mixing until the mixture is glossy and hold stiff peaks. Turn off the mixer and add the meringue to the tops of the tarts, either spreading it with a spatula or using a piping bag to add dollops to the top.
If desired, you can toast the meringue with a kitchen torch. Top the tarts with the citrus slices and sprinkle with the ground pistachio around the edges. Serve chilled. Will keep in the refrigerator in an airtight container for up to 4 days.