In a medium saucepan set over medium-low heat, bring the milk, cream, vanilla bean (including the pod), vanilla extract and the salt just to a boil. Remove from heat; let mixture steep for 30 minutes. While the milk/cream mixture is steeping, heat the sugar with water in a large skillet until it begins to sizzle. Stir in the butter and heat until melted. Add the bananas; cook for about 2 minutes, or until fragrant and softened. Carefully stir in the rum and cook for an additional 2 minutes, or until slightly thickened. Remove from heat. Using a slotted spoon, strain the banana/sugar mixture, reserving the sugar syrup and bananas in separate bowls. Remove the vanilla pod from the steeped milk/cream mixture; discard pod. Stir the lemon juice into the bananas and then mix the bananas into the steeped milk/cream. Cover and refrigerate mixture 1 to 2 hours, or overnight. Turn on the Cuisinart® ice cream maker; pour the banana mixture into the frozen freezer bowl and let mix until thickened, about 15 to 20 minutes. While ice cream is churning, melt the bittersweet chocolate in a bowl set over a pot of simmering water; reserve. When the ice cream is almost fully churned, gradually add the sugar syrup; let mix until fully combined. Once the sugar syrup has been mixed, add the walnuts through the top of the ice cream maker; let mix until combined. Right before the ice cream is done, drizzle the melted chocolate into the churning ice cream. The ice cream will have a soft, creamy texture. If a firmer consistency is desired, transfer the ice cream to an airtight container and place in freezer for about 2 hours. Remove from freezer about 15 minutes before serving.