For the shortbread tart crust:
1. In a large bowl, cream butter, powdered sugar, and orange zest until well combined and fluffy.
2. Add flour, cornstarch, and salt; mix until just combined.
3. Add orange juice and mix until just combined.
4. Butter a 9" tart pan. Divide the shortbread dough into small pieces and distribute it throughout the tart pan. Use your fingers and palm to press the dough into the pan. Try to make it as even as you can on the bottom of the pan and press the dough up into the sides, all the way to the top of the pan. It will take a few minutes to distribute the dough and even it out.
5. Place a sheet of parchment paper over the dough and fill with pie weights or beans. Freeze the dough for at least 20 minutes.
6. Once dough is thoroughly chilled, preheat oven to 375°F. Bake tart dough until golden around the edges and mostly baked through, about 22-27 minutes. Remove parchment paper and pie weights, decrease oven temperature to 325°F, and bake an additional 5-8 minutes, until the bottom crust is fully baked through.
7. Allow to cool completely before filling.
For the hawthorn curd:
1. In a medium metal bowl, whisk together sugar, eggs, hawthorn juice, lemon & orange juice, and salt.
2. Create a double boiler by placing a small pot on the stove with about an inch or two of water at the bottom. Heat over medium heat, placing the bowl of hawthorn curd ingredients on top. The water shouldn't be touching the bowl.
3. Simmer for several minutes, whisking frequently. Cook until hawthorn curd is thick and forms a ribbon when dropped from the whisk, about 7-10 minutes.
4. Turn off heat and whisk in butter.
5. Strain mixture through a fine-mesh sieve into a clean bowl and allow to cool before adding it to the tart crust.
To assemble the tart:
1. Remove cooled tart crust from tart pan and place on a plate or serving platter. Fill with cooled hawthorn curd.
2. Give the tart a gentle jiggle to help smooth out the curd so it forms a smooth, even filling.
3. Allow the tart to chill in the refrigerator for about 20 minutes before decorating. You want to be sure the filling is set so any toppings will stay on top of the curd.
4. Decorate as desired. I piped thin semi-circles of white chocolate on top and placed tiny edible flowers, leaves, and pieces of hawthorn berries on top.
5. Keep tart refrigerated. Once set and decorated, slice and serve.