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1
Make the tart shell: Sift together flour, cocoa powder, and 3/4 teaspoon salt in a medium bowl.
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2
Beat butter and granulated sugar with a mixer on medium speed until pale and fluffy, about 3 minutes.
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3
Beat in egg yolk and vanilla to combine, scraping down side of bowl if needed.
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4
Reduce speed to low, and add flour mixture in 3 additions, alternating with cream and beginning and ending with flour mixture.
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5
Shape dough into a disk, and refrigerate, wrapped in plastic wrap, for at least 30 minutes and up to 2 days.
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6
Preheat oven to 350 degrees.
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7
Roll out dough to a little less than 1/4-inch thickness on a lightly floured surface.
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8
Fit dough into a 9-inch fluted tart pan with a removable bottom.
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9
Trim edge flush with top edge of pan.
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10
Chill in freezer until firm, about 15 minutes.
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11
Prick bottom all over with a fork, and bake until firm, about 20 minutes.
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12
Let shell cool in pan on a wire rack.
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13
Meanwhile, make the roasted oranges:.
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14
Reduce oven temperature to 325 degrees.
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15
Leaving peel on, cut 4 oranges into 1/4-inch rounds, and arrange in a single layer on a rimmed baking sheet.
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16
Pour 1/4 cup juice over oranges, cover tightly with parchment, then foil, and cook until peel is tender, about 2 hours, adding 1/4 cup juice every 30 minutes.
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17
Increase oven temperature to 375 degrees.
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18
Remove foil and parchment, and sprinkle oranges with granulated sugar.
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19
Add remaining 1/4 cup juice, and roast oranges for 15 minutes.
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20
Add 2 tablespoons water, and roast until slightly golden and dry, 2 to 3 minutes more.
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21
Cool completely.
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22
Make the filling: Finely grate zest from remaining orange, about 1 1/2 teaspoons, and stir together with mascarpone(or cream cheese) and confectioners' sugar in a small bowl.
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23
Spread filling in tart shell, and arrange roasted oranges on top, overlapping slightly.
