Preheat oven to 350°. Lightly grease two 9” round cake pans and line the bottoms with parchment paper, then set aside.
Put the crushed pineapple with juice and the banana into a food processor and pulse until blended together. Put the sugar and softened butter into a stand mixer and beat until creamy, about three or four minutes. Whisk together the flour, baking powder, baking soda, salt, and nutmeg, then add into the butter-sugar mixture while beating on medium speed, alternating with the eggs. Add the vanilla extract and beat until you have a creamy batter, scraping the sides of the bowl as necessary, about five minutes.
Divide the batter evenly into the two cake pans and bake in the center of the oven, about 35 minutes or until the cakes are risen, springy to the touch, and a toothpick inserted in the center comes out clean. Remove from the oven and allow to cool in the pans for five minutes before turning out onto a wire rack to cool completely.
While the cakes are cooling, melt the two tablespoons of butter in a nonstick sauté pan over medium heat, then pour in the chopped pecans and toss them in the butter and toast them for a few minutes until they just start to brown and become fragrant. Add the brown sugar and salt and toss well, then set aside to cool.
When the cakes are cool, put the mascarpone and confectioners’ sugar in a bowl and beat with an electric mixer on high for about five minutes until light and fluffy.
To assemble the cake, put one layer, bottom side up, on a serving platter and spread half the frosting evenly across the top. Layer the banana slices over the icing, then top with the other cake layer. Spread the remaining frosting evenly over the top of the cake, then sprinkle the chopped pecans evenly over the entire top layer. Refrigerate for two hours to allow the frosting to set before serving. Will keep in the refrigerator for a week, or in the freezer for up to one month.