Prepare the crust: Preheat oven to 400 degrees with rack in center. Butter a 10-inch tart pan with a removable bottom; set aside. In a medium bowl, whisk to combine flour, sugar, and salt. In a small glass measuring cup, stir together butter, vanilla, and 1/2 tablespoon water; pour over flour mixture. Stir to combine. With floured hands, press dough into prepared pan. Bake for 10 minutes. Remove tart shell from oven, and reduce heat to 350 degrees.
Prepare the filling: In a large bowl, stir together almonds, heavy cream, sugar, egg, almond extract, Grand Marnier, and salt. Pour filling into crust. Bake tart on a parchment paper-lined baking sheet until golden brown, about 45 minutes.