• Preheat oven to 350 F (180 C) for a conventional oven. Lower the temperature by 20 degrees for a convection oven
• Spray the baking pan, 10 x 15 x 1 inch, with non-stick cooking spray or brush some oil and then line with a parchment paper on the bottom and the side

Make the Banana Buttercream

• Place white chocolate chips in a heatproof bowl and microwave in 15-second intervals, stirring between each, until fully melted and smooth. Alternatively, melt over a double boiler. Let it cool before mixing it with the butter
• In a mixing bowl, beat softened butter and icing sugar on high speed for 1-2 minutes until light and fluffy.
• Add the melted white chocolate, mashed banana, vanilla, and salt. Continue beating for 3-4 minutes until smooth and airy. Add food coloring if using.
• This can be prepared a few days before,covered, and chill in the fridge. When you are ready to use it, let it sit at room temperature until it is softened again. You can also whip it again to make it light and fluffy befoe using

Prepare the yolk batter:

• In a mixing bowl, whisk together the mashed banana, egg yolks, oil, and vanilla
  
• Then add cake flour, baking powder, and salt, and whisk until combined
  

Prepare the meringue:

• Make sure the egg whites are at room temperature. Place egg whites in a mixing bowl of a stand mixer fitted with a whisk attachment. Whisk until foamy on medium speed and then add cream of tartar or lemon juice/vinegar and whip again for another minute
  
• Add 1/3 of the sugar and whip for 1 minute on speed 6 (on KA machine) and continue to add sugar in 1-minute increment until you use it all up.
  
• Whip until the egg white develops a medium-firm peak. When you lift the meringue up it will hold up its shape but the tail will bend. Do not whip until stiff peak. The cake will crack when you roll it later
  

Combine meringue with cake batter:

• Fold about 1/3 of the meringue into the egg yolk batter
  
• Use a rubber spatula or whisk to fold until they are combined. Then fold the next 1/3 and fold again until combined.
  
• Add the last 1/3 of meringue and continue to fold,making sure both yolk batter and the meringue is well-combined. The batter is voluminous, smooth, and light.
  

Pour into a prepared baking pan:

• Pour the batter into the pan
  
• Use an offset spatula to help spread evenly. You can use a dough scraper to help you even out the surface too if necessary
  

Bake:

• Gently tap the baking sheet on your working surface several times to pop any air bubbles. Bake in a preheated oven for 25 minutes and then switch to convection mode (fan mode) and bake for another 3-4 minutes or until the top is golden brown and dry to touch. If you don't have convection mode, bake another 5 minutes and check to make sure the top is golden brown and dry to touch.
  
• It is important that the top is dry to touch so this layer won't peel off when you roll later
  

Cool down:

• Remove the cake from the oven and then lift it out from the pan immediately to a cooling rack and peel off the parchment paper from 4 sides. This helps to prevent wrinkles on the surface. Let it cools down like this for 10 minutes (I use a timer to actually time it). While waiting, prepare the butter cream if you haven't already.
  

Assembling the cake:

• Place a new piece of parchment paper, larger than the cake, on top of the cake and flip over to the other side. Peel off the parchment paper
  
• Use a serrated knife to trim about 1/4-inch of the 4 edges. Use a serrated knife to make 4 shallow slits along the long side of the cake, about 1 inch apart, taking care not to cut through the cake. This makes rolling the cake easier. This photo is from the other Swiss roll that I baked. Just to show you how the shallow slits is cut
  
• Spread the banana buttercream. You want to spread most of the filling near the side you are going to roll up from and there should be less filling towards the other end because as you roll up, the filling will get pushed forward and by the time you roll to the other end, it is just a perfect amount of filling
  
• I sprinkled some mini chocolate chips. This is optional. It's just my last-minute decision to include them
  
• Very gently use one hand to lift the parchment paper up to help you roll the cake up until you reach the other side.
  
• Tuck the parchment paper under and use a dough scraper to help you tighten the roll-up
  
• Remove parchment paper so the skin of the cake won't peel off later when you remove the parchment paper.

Make the Dark Chocolate Ganache

• Heat the heavy cream in a small saucepan until it just starts to simmer. Pour over the chocolate and let sit for 2-3 minutes
  
• Then stir until smooth.
  
• Pour the ganache over the chilled Swiss roll, smoothing it over the surface.
  
• Sprinkle with a bit of flaky sea salt if you want
  

Chill & Serve

• Refrigerate overnight for the best texture and flavor.
• Slice the cake using a serrated knife. Wipe the knife clean after each cut. I forgot to do so and you can see that I didn't get a very clean cut.
• The cake is best served chilled, allowing the banana flavors to meld beautifully with the chocolate.