Heat oven to 325°F and arrange rack in the middle. Grease a 10-inch cake pan and line a baking sheet with parchment paper.
Combine 1/2 cup dulce de leche, allspice, nutmeg, and rum in a small bowl and whisk until smooth. Pour into cake pan and smooth. Arrange banana slices on top.
Combine flour, espresso, baking soda, and salt in a medium bowl and whisk break up any lumps.
Beat butter and sugar in the bowl of a stand mixer fitted with the paddle attachment until light and fluffy, about 3 minutes.
Add eggs, one at a time, letting the first one incorporate before adding the next. Add sour cream and beat until fully combined, scrapping down the sides of the bowl with a rubber spatula as needed.
Add flour mixture and walnuts and mix on low, just until flour is fully incorporated, about 1 minute.
Pour batter on top of bananas and smooth top. Place cake pan on baking sheet (to catch any drips) and bake until cake is set, about 1 hour. Cake will be done when a toothpick comes out with a few crumbs and the cake is firm in the middle.
Let cake cool on a wire rack about 15-20 minutes. While cake is cooling, heat remaining dulce de leche over low heat, stirring until melted and pourable. Run a butter knife around the edge, then invert onto a cake stand or serving plate. Drizzle top with dulce de leche and serve.