Instructions

for the filling

Whisk yolks in medium saucepan; gradually whisk in evaporated milk. Add sugars, butter, and salt and cook over medium-high heat, whisking constantly, until mixture is boiling, frothy, and slightly thickened, about 6 minutes. Transfer mixture to bowl, whisk in vanilla and rum, then stir in coconut. Cool until just warm, cover with plastic wrap, and refrigerate until cool or cold, at least 2 hours or up to 3 days. (Macadamia nuts are stirred in just before cake assembly; banana slices are added during assembly.)

for the cake

Adjust oven rack to lower-middle position; heat oven to 350 degrees. Combine chocolate and cocoa in small bowl; pour boiling water over and let stand to melt chocolate, about 2 minutes. Whisk until smooth; set aside until cooled to room temperature.
Meanwhile, spray two 9-inch-round by 2-inch-high straight-sided cake pans with nonstick cooking spray; line bottoms with parchment or waxed paper rounds. Spray paper rounds, dust pans with flour, and knock out excess. Sift flour and baking soda into medium bowl or onto sheet of parchment or waxed paper.
In bowl of standing mixer, beat butter, sugars, and salt at medium-low speed until sugar is moistened, about 30 seconds. Increase speed to medium-high and beat until mixture is light and fluffy, about 4 minutes, scraping down bowl with rubber spatula halfway through. With mixer running at medium speed, add eggs one at a time, beating well after each addition and scraping down bowl halfway through. Beat in vanilla; increase speed to medium-high and beat until light and fluffy, about 45 seconds. With mixer running at low speed, add chocolate, then increase speed to medium and beat until combined, about 30 seconds, scraping down bowl once (batter may appear broken). With mixer running at low speed, add dry ingredients in 3 additions, alternating with sour cream (in 2 additions), beginning and ending with dry ingredients, and beating in each addition until barely combined. After final flour addition, beat on low until just combined, then stir batter by hand with rubber spatula, scraping bottom and sides of bowl, to ensure that batter is homogenous (batter will be thick). Divide batter evenly between prepared cake pans; spread batter to edges of pans with rubber spatula and smooth surfaces.
Bake cakes until toothpick inserted into center of cakes comes out clean, about 30 minutes. Cool in pans 10 minutes, then invert cakes onto greased wire rack; peel off and discard paper rounds. Cool cakes to room temperature before filling, about 1 hour. (Cooled cakes can be wrapped in plastic wrap and stored at room temperature for up to 1 day.)

to assemble

Stir toasted macadamia nuts into chilled filling. Set one cake on serving platter or cardboard round cut slightly smaller than cake, and second cake on work surface (or leave on wire rack). With serrated knife held so that blade is parallel with work surface, use sawing motion to cut each cake into two even layers. Starting with first cake, carefully lift off top layer and set aside. Arrange one quarter of banana slices on cake layer, then using icing spatula, distribute about 1 cup filling evenly on cake, spreading filling to very edge of cake and leveling surface. Carefully place upper cake layer on top of filling and bananas; repeat using remaining bananas, filling, and cake layers. If necessary, dust crumbs off platter; serve or refrigerate cake, covered loosely with foil, up to 4 hours (if refrigerated longer than 2 hours, let cake stand at room temperature 15 to 20 minutes before serving).