Spray an 8" cake pan with cooking spray, then dust with flour. Invert pan and knock out excess. Reserve.
In the bowl of a stand mixer, beat eggs on medium speed with whip attachment until thick and fluffy. This should take several minutes.
In the meantime, combine the flour, Splenda, baking soda, baking powder and salt in a sifter, and sift ingredients into a large bowl. Reserve.
When eggs are thick and fluffy, add the vanilla and continue to beat for a few moments. Add the Canola oil and continue mixing for 30 seconds.
Now, stop the mixer and replace the whip with the paddle attachment. Add the banana puree and mix on low-medium speed until incorporated with egg mixture, about 30 seconds.
Finally, lower mixer to stir speed (slowest) and add 1/3 of the flour mixture, then 1/2 of the buttermilk. As soon as these are incorporated (10 seconds?) add another third of the flour mixture and then the rest of the buttermilk. After another 10 seconds, add the remainder of the flour mixture and mix for a few seconds. Then turn the mixer off.
Gently spoon the batter into the pan and bake at 350 on the center rack for 20-25 minutes, until a tester inserted in the center comes out dry. Don't overbake!
I must say that I have only make this as muffins - the recipe makes 6 large muffins. I have not made this in a cake pan, so I can't say exactly how long to bake it. As muffins, in my oven, 25 minutes has been the magic number. Also, when the muffins are done I let them cool completely in the muffin tin. Then run a knife around the edges to loosen and remove.
Another touch I add is a light sprinkling of shreded coconut on top of the batter before placing the tin in the oven. The coconut seems to brown around the time the muffins are ready to come out of the oven.