To make crust, heat oven to 350°. Combine cookie crumbs, butter, and salt in a large bowl until mixture resembles wet sand. Firmly and evenly press mixture into 8-inch tart pan. Bake until fragrant, about 15 minutes. Let cool completely while preparing filling.

To make custard, whisk together ¼ cup milk with cornstarch in a medium bowl. Add eggs and whisk to combine. Combine remaining 1¾ cups milk, sugar, and salt in a medium, heavy-bottomed saucepan. Bring to a boil over medium heat. Remove from heat and ladle about ¼ cup hot milk mixture into bowl with egg mixture, whisking constantly. Slowly whisk egg mixture into saucepan and return to low heat. Whisk continuously until mixture becomes very thick, about 1 minute. Remove from heat and whisk in butter and vanilla.

Transfer custard into tart shell and smooth top. Chill for 1 hour while preparing topping.

To make topping, carefully cut all the peel and pith from the oranges. Thinly slice fruit crosswise and pop out any seeds. Arrange slices on top of custard in concentric, slightly overlapping circles.

Heat marmalade in a small saucepan over medium heat until melted. Brush warmed marmalade on fruit just until shiny.