Place oven rack in bottom third of oven and preheat the oven to 350F; line a 9 X 3 inch round cake pan or springform pan with wax paper and grease the paper; (If using a springform pan that doesnï¿½t seal tightly, wrap the bottom with foil to prevent leakage).
Make the banana cake: Sift together the flour, baking soda, baking powder and salt; in a large bowl, cream the butter with the sugar at medium speed until smooth; add the eggs one at a time, beating for 1 minute after each addition; in a bowl, mash the bananas with the buttermilk; beat the dry ingredients into the butter at medium speed in two additions, alternating with the bananas (the batter may appear slightly curdled ï¿½ thatï¿½s OK); Pour the batter into the prepared pan and smooth the top; bake in the bottom third of the oven for about 1 hour, or until it tests done; (MAKE AHEAD: The cake can be made 1 day ahead; wrap in plastic wrap and store at room temperature.).
Make the banana buttercream filling: In a large bowl, beat the egg yolks and confectionersï¿½ sugar at high speed until pale and thick; meanwhile, in a medium saucepan, bring the milk and banana to a boil over medium heat, whisking occasionally; slowly whisk the hot milk into the egg yolks; pour the mixture back into the saucepan and cook over medium-low heat, stirring, until thick, about 5 minutes; do not boil; transfer the custard to a medium bowl and let cool briefly, whisking occasionally; then press a piece of plastic wrap directly on the surface of the custard and refrigerate until chilled, atleast 3 hours or overnight; bring close to room temperature before continuing, otherwise, your buttercream will curdle. Beat the butter at high speed until fluffy; Reduce the speed to low and gradually mix in the cold banana custard; beat at medium speed until fluffy, then beat in the espresso.
Assemble the cake: Split the cake into 3 equal layers (they will be quite thin); place one layer on a cake plate and cover with half of the banana buttercream filling; top with a second cake layer and the remaining filling; place the third cake layer on top, cover with plastic wrap, and refrigerate.
Make the coffee frosting: In a large, heatproof bowl, beat the sugar and egg whites until mixed; place the bowl over a large saucepan of simmering water; do not let the bowl touch the water; heat the mixture, whisking occasionally, until the sugar is dissolved, about 5 minutes; remove from the heat and beat at high speed until fluffy, about 3 minutes; Beat the meringue at medium speed until cool to the touch, about 5 minutes; Beat in the butter several pieces at a time, waiting until they are incorporated before adding more; whip the frosting until fluffy; (if the frosting appears lumpy, beat at high speed until smooth; If the mixture is soft and runny, refrigerate for 6-8 minutes, then beat again) beat in the espresso and vanilla until blended; set aside ½ cup of the frosting for decorating; spread the remaining frosting on the sides and top of the cake; with the reserved frosting, using a pastry bag fitted with a star tip, pipe 8 rosettes on the top edge of the cake; Place 1 chocolate-covered espresso bean in the center of each rosette; to serve: Slice gently with a serrated cake knife (MAKE AHEAD: the cake can be refrigerate for 1 day; let stand at room temperature for 1 hour before serving).