1. Whisk together first 3 ingredients in a large heavy saucepan. Gradually whisk in milk and cream. Cook over medium heat, stirring constantly, 10 to 12 minutes or until mixture thickens slightly. Remove from heat.
2. Whisk egg yolk until slightly thickened. Gradually whisk about 1 cup hot cream mixture into yolk. Add yolk mixture to remaining cream mixture, whisking constantly. Whisk in vanilla bean paste. Cool 1 hour, stirring occasionally.
3. Meanwhile, preheat oven to 400°. Combine bananas, brown sugar, butter, and, if desired, 1 Tbsp. banana liqueur in a 2-qt. baking dish. Bake 20 minutes or until browned and softened, stirring after 10 minutes. Let cool 30 minutes.
4. Coarsely mash banana mixture; stir into cooled cream mixture. Place plastic wrap directly on cream mixture, and chill 8 to 24 hours.
5. Pour mixture into freezer container of a 1 1/2-qt. electric ice-cream maker, and freeze according to manufacturer's instructions. (Instructions and time may vary.) Before transferring ice cream to an airtight container for further freezing, stir in vanilla wafers and, if desired, remaining 1 Tbsp. banana liqueur.
*Vanilla extract may be substituted.