In a food processor, pulse the flour, sugar, and salt a few times to combine. Add the butter and orange zest and pulse until the mixture resembles cornmeal, six to eight 1-second pulses. A teaspoon at a time, pulse in up to 1 tablespoon water until the dough just holds together in clumps. Press the dough together, shape into a 6-inch disk, and wrap in plastic. Refrigerate for 30 minutes.

Press the dough evenly into the bottom and sides of a 9-1/2-inch fluted tart pan with a removable bottom-the dough sides should be 1/4 to 3/8 inch thick. To smooth the bottom, cover with plastic wrap and press with a flat-bottom measuring cup or glass. Freeze the covered shell for 30 minutes. Meanwhile, position a rack in the center of the oven and heat the oven to 400 degrees F.

Remove the plastic, line the dough with parchment and fill with dry beans or pie weights. Bake the tart shell until the top edges are light golden, about 15 minutes. Carefully remove the parchment and beans, reduce the heat to 375 degrees F, and continue to bake until the shell is golden all over, about 15 minutes. Cool on a rack.

In a small saucepan, heat the butter over medium-high heat until it melts and the milk solids turn brown, swirling the pan occasionally for even browning, about 3 minutes. Immediately pour into a small heatproof bowl to stop the cooking.

In a medium bowl, whisk 1/4 cup of the milk with the cornstarch. Whisk in the eggs.

In a medium saucepan, bring the remaining 1-3/4 cups milk, the sugar, and salt to a boil over medium heat. Take the pan off the heat, whisk about 1/4 cup of the hot milk into the egg mixture, and then whisk the egg mixture into the hot milk. Return to medium heat and continue whisking until the filling boils and becomes very thick, 30 seconds to 1 minute. Off the heat, whisk in the brown butter and vanilla.

Spread the filling evenly in the tart shell and set aside at room temperature while you prepare the topping.

Using a sharp knife, trim off the peel and pith from the oranges. Halve the oranges lengthwise and then slice them thinly crosswise and remove any seeds. Arrange the orange slices on the top of the tart in concentric, slightly overlapping circles.

Stir the marmalade in a small saucepan over medium heat until melted, 30 to 60 seconds. Strain and then stir in the Cointreau. Brush enough of the mixture on the oranges to give them a shine (you may not need it all). Refrigerate for 1 hour before serving so the filling can set up.