To prepare filling, pour 2 cups milk into a medium, heavy saucepan. Scrape seeds from vanilla bean; add seeds and bean to pan. Cook milk over medium-high heat to 180° or until tiny bubbles form around edge (do not boil). Remove from heat.
Combine 1/2 cup granulated sugar, cornstarch, dash of salt, and egg yolks, stirring with a whisk. Gradually add 1 cup hot milk mixture to egg mixture, stirring with a whisk. Return milk mixture to pan; bring to a boil over medium heat, stirring constantly with a whisk. Cook 1 minute; remove from heat. Spoon into a bowl; discard vanilla bean. Stir in sour cream. Place plastic wrap on surface of custard. Chill thoroughly.
To prepare crepes, combine 1/2 cup milk and next 4 ingredients (through egg) in a blender. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Add flour, 1/2 teaspoon granulated sugar, and 1/4 teaspoon salt. Process until smooth. Chill 1 hour.
Heat an 8-inch nonstick crepe pan or skillet over medium heat. Coat pan lightly with cooking spray. Pour about 3 tablespoons batter into pan; quickly tilt pan in all directions so batter covers pan with a thin film. Cook about 2 minutes. Carefully lift the edge of the crepe with a spatula to test for doneness. Turn crepe when it can be shaken loose from the pan and the underside is lightly browned; cook 1 minute or until center is set.
Place crepe on a towel; cool completely. Repeat procedure until all of batter is used. Stack crepes between single layers of wax paper to prevent sticking.
To prepare remaining ingredients, combine citrus sections, 1/4 cup granulated sugar, and chopped mint; toss gently to dissolve sugar.
Place 1 crepe on each of 8 dessert plates; spread about 1/3 cup chilled filling over each crepe. Fold each into a triangle. Top each serving with 1/4 cup fruit mixture. Sprinkle crepes evenly with powdered sugar. Garnish with mint sprigs, if desired.