Preheat oven to 350°. Bake pecans in a single layer in a shallow pan 6 to 8 minutes or until toasted and fragrant, stirring halfway through.
Coat 3 (9-inch) round cake pans with cooking spray. Line bottom of each pan with parchment paper; coat paper with cooking spray.
Whisk together flour and next 5 ingredients in a large bowl. Add eggs, oil, and vanilla, and stir just until dry ingredients are moistened. Fold in bananas, pineapple, and pecans. (Batter will be very thick, more like banana bread batter than cake batter.) Spoon batter into prepared pans.
Bake at 350° for 28 to 30 minutes. Cool in pans on a wire rack 10 minutes. Remove from pans to wire rack, and cool completely (about 30 minutes). Peel off and discard parchment paper.
Place 1 cake layer on a cake stand or serving plate. Top with one-fourth of frosting. Repeat procedure once. Top with remaining cake layer. Spread remaining frosting over top and sides of cake. Garnish, if desired.