Make and refrigerate the tarts (without the raspberries) earlier in the day. Arrange the fruit on top just before serving. You can also make the tarts in 2 (6-ounce) ramekins or custard cups.
Preheat oven to 350°.
To prepare crust, combine crumbs, butter, and 1 teaspoon sugar, tossing with a fork until moist. Divide mixture evenly between 2 (75-inch) ceramic crème brûlée dishes coated with cooking spray. Press mixture into bottom of dishes. Bake at 350° for 10 minutes or until lightly browned; cool completely on a wire rack.
To prepare filling, combine 1/4 cup milk, 3 tablespoons sugar, cornstarch, salt, and egg yolk in a medium bowl, stirring with a whisk. Heat 3/4 cup milk over medium heat in a small, heavy saucepan to 180° or until tiny bubbles form around edge (do not boil). Gradually add hot milk to sugar mixture, stirring constantly with a whisk. Place mixture in pan; cook over medium heat until thick and bubbly (about 5 minutes), stirring constantly.
Remove pan from heat; stir in liqueur and vanilla. Spoon custard into a small bowl; place bowl inside a medium ice-filled bowl for 10 minutes or until custard comes to room temperature, stirring occasionally. Remove bowl from ice; spoon custard into prepared crusts. Cover and chill 2 to 6 hours.
Uncover tarts. Arrange 1/4 cup raspberries on top of each tart. Garnish with mint sprigs, if desired.