Preheat oven to 350ºF. Spray three 9 inch cake pans with cooking spray. Line each bottom with a cut-to-size parchment or wax paper circle. Spray tops of paper and set aside. Tip: If you only have 2 9" cake pans, bake 2 layers, and store remaining batter covered on counter. Bake 3rd layer in COOLED cake pan.
In a small bowl, mix mashed bananas with lemon juice and set aside. In a medium bowl whisk together flour, baking soda, baking powder and salt. Set aside.
In a large bowl cream 3/4 cup butter with sugar until light and fluffy, at least 3 minutes. Beat in the egg yolks one at a time, then mix in vanilla and (optional) banana extracts. Add about 1/3 of the flour mixture and alternate with about 1/3 buttermilk. Repeat until all the flour and buttermilk has been mixed in. Mix in banana mixture.
In a separate clean, dry bowl beat the reserved egg whites until stiff. Fold 1/3 of the egg whites into the banana batter, then gently fold in the remaining 2/3 until no streaks remain. Divide the batter between the prepared pans.
Bake for 30-33 minutes (about 5 minutes longer if using gluten free flour) or until toothpick inserted at center comes out clean. Turn onto wire racks and cool completely.
Place all the banana filling ingredients in a blender or food processor and blend until fully incorporated and smooth. If you have an immersion blender, put everything in medium saucepan and blend it in there.
Pour mixture into medium saucepan over medium-low heat. Stir frequently until the mixture becomes very thick and starts to boil, about 10-15 minutes. If the filling starts to boil before it is thick enough, lower the heat more. When the filling starts to cleanly pull away from sides of pan while stirring it is done (see pic in post).
Remove from heat and cool completely. I recommend chilling the filling until cold so it sticks to the cake layers and doesn't slide around. Makes 2 cups.
Whipped Cream Frosting:
Place the sugar and cornstarch in a small saucepan. Measure 1 1/2 cups of heavy cream and remove 6 tablespoons, adding it to the saucepan. Place the remaining measured cream in the refrigerator. Heat the sugar, cornstarch, and 6 tablespoons of heavy cream over medium heat, whisking constantly and bring to a boil. Remove from the heat, stir in the vanilla, and let cool completely, about 45 minutes (can refrigerate to speed up process, but stir occasionally).
After the cornstarch mixture has cooled beat the remaining chilled cream until tracks begin to show, about 2 minutes. Add the cornstarch mixture and continue beating until stiff peaks form, about 1 to 2 minutes more.
Place cake layer on serving platter and pour 1/2 the banana curd filling in the middle and smooth out until it reaches the edge. Gently place 2nd layer on top and repeat. Place top layer on and smooth whipped cream all over the outside of the cake.
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Recipe for Chocolate Whipped Cream FrostingMake ahead instructions: You can make the cake layers ahead, cool completely, wrap tightly and freeze until you are ready to assemble the cake.The Banana Filling will stay for a week in the fridge, stored in an airtight container.The regular whipped cream frosting is very stable and can be stored covered in the fridge for a couple days in an airtight container.Banana Filling adapted from iambaker.net