Preheat oven to 350F and lightly grease two round 9-in baking pans. Set aside.
Make the cake: In a medium bowl, combine flour, cornstarch, baking powder, and salt. In a large bowl, whisk mashed banana, xylitol, eggs, oil, crushed pineapple, and vanilla until smooth. Gently fold flour mixture into liquid until smooth and combined. Fold in toasted pecans.
Scrape batter into prepared pans and smooth tops. Bake cakes at 350F 23-25 minutes, or until a toothpick comes out clean. Cool cakes completely on a wire cooling rack.
Make the frosting: Place softened cream cheese and butter in a medium bowl. With an handheld electric mixer, beat on medium speed until creamy. Beat in powdered xylitol, vanilla extract, and enough milk to achieve a spreading consistency (add a little more milk if frosting seems too thick)
Place one cooled cake layer on a serving plate and fill with about 1/3 of frosting. Place remaining cake layer on top, then frost top and sides of the cake with remaining frosting. Garnish cake with additional toasted pecans if desired.