Preheat oven to 350 degrees. Prepare 2 - 7" round pans. Use a pastry brush to apply Cake Release to the bottom and sides of pans.
Using a whisk, combine flour, baking soda, baking powder and salt in a bowl.
In a separate bowl combine mashed bananas, buttermilk, and vanilla.
Add butter and sugar to the bowl of your stand mixer and using the paddle attachment, beat until light and fluffy.
Add the eggs one at a time and continue to beat. Scrape down the sides of the bowl as needed.
Add the flour mixture in thirds, alternating with the banana mixture - ⅓ flour mixture, ½ banana mixture - repeat, ending with the flour mixture. Beat until fully incorporated.
Remove the bowl from the mixer and grate the chocolate into the batter. Fold in the chocolate using a spatula and divide the batter evenly between the two pans.
Bake for 25-30 minutes, or until an inserted toothpick comes out clean.
Remove from the oven and allow the cakes to cool on racks for about 10-15 minutes.. Invert on a wire rack to cool completely, if using immediately. Or wrap in plastic wrap while still warm and store in freezer until needed.
To make the sauce:
Place butter, brown sugar and heavy cream in a saucepan over medium heat. Stir until the mixture is smooth. Once it is smooth stop stirring. Let the mixture cook at a bubbling simmer for 3 minutes. Remove the mixture from the heat. Stir in the remaining cream. Let the mixture cool. Stir in vanilla extract. Store butterscotch in a mason jar in the refrigerator.
To make the frosting:
Using a clean towel and some lemon juice or vinegar, clean all equipment and utensils to be used for preparing the meringue.
Fill a heavy duty pot about a quarter of the way with water and place on the stove over medium heat to simmer.
Add egg whites, sugar and salt to the mixing bowl and use a hand whisk to combine. Attach a candy thermometer to the side of the bowl and place the bowl over the pot, ensuring the bottom of the bowl is not in contact with the water in the pot. Lightly whisk the sugar egg white mixture constantly until temperature reaches 160°F. This took me about 12 minutes.
Once at the correct temperature, remove from heat, wipe away condensation from the bottom of the bowl with a towel and transfer mixing bowl to the stand mixer. With the whisk attachment, and the mixer set to high (8-10), whip the meringue until it is thick, glossy, and the bottom of the bowl is no longer warm to the touch. Do not begin adding butter while the bowl is still warm. This took about 10 minutes.
Switch from your whisk to paddle attachment and with mixer on low speed, add butter cubes, one at a time, until incorporated. Continue to mix until it has reached a silky smooth texture. This took approximately 12 minutes.
Add emulsion and vanilla beans and continuing to beat on low speed until well combined. About 2 minutes.
Assembly: If frozen, remove cakes from freezer a few hours ahead of assembling, wrap in tea towels and allow to defrost at room temperature. Level cakes, once defrosted.
Using a cake board just slightly larger than the size of your cake, spread a small amount of frosting over the board and place the first layer of banana cake on the board, centering it.
Fit a #12 round decorating tip into a piping bag and will with about half cup of buttercream. Pipe a dam around the cake, just inside the edge and continue in circles, ending in the middle. Use a small offset spatula to smooth the buttercream.
Add 2-3 spoonful of butterscotch onto the buttercream and spread with an offset spatula.
Place another banana cake layer on top of frosting and press lightly, but firmly until levelled. Repeat as above. Add the chocolate layer and repeat as above for all the remaining layers.
Use any leftover frosting in the piping bag to fill in any gaps between layers and use an offset spatula to seal the seams.
Add a thin layer of frosting over the entire cake to seal in the crumbs and place in the fridge for 15-20 minutes.
Remove from the fridge and frost the cake, using the cake board as your guide for applying an even coat of frosting. Smooth out the frosting using a spatula. Decorate to your liking.
Best enjoyed at room temperature!
Notes
[cake and sauce recipes adapted from The Baked Equation]