How to Make It
Preheat the oven to 350°. Line a 9-by-3-inch round cake pan or springform pan with wax paper and grease the paper. (If using a springform pan that doesn't seal tightly, wrap the bottom in foil to prevent leakage.)
Sift together the flour, baking soda, baking powder and salt. In the large bowl of a standing electric mixer, cream the butter and 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.)
Scrape the batter into the prepared pan and smooth the top. Bake in the bottom third of the oven for about 1 hour, or until a cake tester inserted in the center comes out clean.
In a large bowl, using an electric mixer, beat the egg yolks and confectioners' sugar at high speed until pale and thick. Meanwhile, in a medium saucepan, bring the milk and banana to a boil over moderate heat, whisking occasionally. Slowly whisk the hot milk into the egg yolks. Pour the mixture into the saucepan and cook over moderately 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 wax paper directly on the surface and refrigerate until chilled, at least 3 hours or overnight.
In a bowl, using an electric mixer, 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.
Split the cake into 3 layers (they will be relatively thin). Place one layer on a cake platter and cover with half of the buttercream filling. Top with a second layer and the remaining filling. Place the third layer on top, cover with plastic wrap and refrigerate.
In a large heatproof bowl, using an electric mixer, 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 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 5 minutes, then beat again.) Beat in the espresso and vanilla until blended.
Set aside 1/2 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.