To bake the cake:
Preheat oven to 350degreesF.
Butter and flour an 8-inch cake pan and an 8-inch metal mixing bowl. Line the cake pan with a parchment paper round. Set aside.
In a medium bowl whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a mixer fitted with the paddle attachment, cream together the butter and sugar until light in color and fluffy. Mix in the honey and vanilla. Add the eggs one at a time, mixing well after each. Add the banana puree and mix until incorporated.
Alternate adding the flour mixture and buttermilk, starting and ending with the dry ingredients.
Divide the batter between the cake pan and the prepared bowl.
Bake the cakes for about 40 minutes, or until a tester comes out clean. The cake in the bowl may take a few more minutes. Remove and allow to cool completely.
To make the buttercream:
In the bowl of a stand mixer combine the egg whites and sugar. It will be very thick and grainy. Put the bowl over a double boiler and stir it with a rubber spatula until the sugar is completely melted. Brush the sides down with the spatula to make sure all the sugar is melted. Feel the egg mixture between your fingers to check for graininess. Once it is completely smooth, put the bowl on the mixer and beat with the whip attachment on medium high speed.
Whip until it is light, fluffy, glossy and the bowl feels just about room temperature. If the egg whites are not cooled off sufficiently it will melt the butter when you add it.
Once the egg whites are whipped and cooled, add the butter, 2 tablespoons at a time on medium speed.
WARNING: After you have added about half of the butter, it may look curdled and runny, this is normal and you should continue adding the rest of the butter.
Once you have finished adding the butter and it has mixed on medium speed for about a minute the buttercream will be creamy and glossy looking again. Add the honey, salt, and vanilla.
Divide the buttercream in half and add the pecans to one batch and the yellow food coloring to the other.
Cut each of the cake layers in half. Start with the cake pan layers and fill them with the walnut buttercream. Repeat with the rounded layers baked in the metal bowl.