Heat the oven to 350°F and place a rack in the center.
Toast the nuts: On a baking sheet, toast the walnuts or pecans for 8-10 minutes until fragrant. Remove from the oven, stir in 2 teaspoons of salt and butter, and mix to combine. Let cool before finely chopping.
Prepare the pans: Butter and flour three 9-inch cake pans and line them with parchment paper.
Mix dry ingredients: In a medium bowl, whisk together the sifted flour, baking powder, baking soda, cinnamon, and remaining 1 teaspoon salt. Set aside.
Whip eggs and sugar, add the oil: In a stand mixer, whip sugar and eggs on medium-high speed until the mixture is light and fluffy. Turn the mixer to low and slowly drizzle in the vegetable oil. Combine wet ingredients: Fold in the mashed bananas, pineapple, sour cream, and vanilla extract.
Mix dry and wet ingredients: Gradually add the flour mixture and chopped nuts, mixing until just combined. Be careful not to overmix.
Bake: Divide the batter evenly among the prepared pans. Bake for 25-30 minutes, until golden brown and springy to the touch. Allow to cool completely. Optionally, brush with Malibu rum while the cake is still warm.