We start with the oranges. Slice 3 of the oranges into 1/4 inch thick rounds. You can leave the peel or remove before slicing.
Preheat the oven to 325℉
Line a baking sheet with parchment paper and arrange the slices on it barley overlapping each other if you need to.
Pour 1/4 cup of orange juice over the slices and cover with a parchent and then with foil.
Cook for 2 hours. Pouring 1/4 cup of orange juice on the sheet every 30 minutes.
After 2 hours is up set the oven temp to 375℉. Remove the foil and top parchement paper. Pour on the last 1/4 cup of water and sprinkle the oranges with 1/3 cup sugar. Cook for 15-20 minutes or until the oranges begin to look golden.
Remove from the oven and let them cool completely.
In the meantime while the oranges are cooking, start the shell.
In a medium bowl add the flour, salt, and cocoa powder.
In a standing mixer or with a hand mixer and large bowl, beat the butter and sugar until fluffy. Add the vanilla extract and egg yolk. Mix well.
On low speed alternate adding the flour mixture and heavy cream until all incorperated.
Press the dough into a ball and then a disc shape. Cover in plastic wrap and refrigerate for 30 minutes or until the oranges are done.
Preheat the oven to 350℉
Lightly flour a work space and roll out the dough into about 1/4 inch thickness. If it is a bit crumbly that is ok, just pinch it back to shape.
Fit into the tart pan, trimming off any excess edges.
Freeze for 15 minutes. Then remove and prick the bottom with a fork. Bake for 20 minutes and let cool. While that cools lets mix up the filling. In a bowl combine the mascarpone, orange zest, and powdered sugar. Spread into the tart pan evenly.
Top with the orange slices.
Sprinkle on a bit of granulated sugar onto the slices and powdered sugar over all of the tart.
Slice and ENJOY!
Can be stored room temperature for 2 days.