Orange and spice flavors combine perfectly in this luscious dessert. The gingersnap crust can be made ahead and frozen for up to 2 weeks, and the Pastry Cream filling can be made up to 8 hours ahead. Sectioning the oranges takes extra time – about 15 minutes – but yields a far superior result.
Preheat oven to 325 degrees F. Use 1 tablespoon butter to coat a 9-½-inch tart pan (with removable bottom). To make the crust: Melt remaining 5 tablespoons butter over low heat. Grind gingersnaps in a food processor until they’re the texture of sand, then pour into mixing bowl. Pour melted butter over gingersnap crumbs and work together with your hands; press crust into sides and bottom of the tart pan. Place tart crust on baking sheet and refrigerate 20 minutes. Transfer to oven and bake about 15 minutes, until lightly browned. Cool completely.
Meanwhile, prepare Pastry Cream. While it is still warm, stir in orange zest and Grand Marnier. Press plastic wrap against entire surface of cream, and let cool to room temperature. To prepare the topping: Slice top and bottom off each orange. Starting from the top and following the shape of the fruit, cut away all the pith and peel. Holding the orange over a bowl, cut away each section from the membrane. You should be left with a wedge-shaped slice of pure fruit and a bowl of juice (save it to drink at breakfast). Drain orange slices on paper towels. While slices are draining, heat marmalade in a small saucepan until it is liquid. Remove from heat and strain through a mesh strainer; discard pulp. Add cinnamon to marmalade liquid and set aside.
To assemble the tart: Pour cooled Pastry Cream into tart shell. Spread evenly. Lay orange slices sideways around tart, layering each successive row on top of the previous row. Brush fruit with marmalade glaze. Chill. This dessert is best when served within 6 or 8 hours.