Preheat the oven to 350 F. Grease 8 x 8 x 2 inch square cake pan or 9 x 2 inch round cake pan. The pan must be 8 cups volume (cake will overflow 8 x 8 x 1 1/2 inch square pan, it has only 6 cup volume)
Spread 2/3 cup brown sugar on the bottom of the pan and scattered pieces of butter (4.5 Tbsp). Place the pan in the oven for 5-7 minutes (until butter has melted). Remove from the oven and stir butter and sugar and spread evenly on the bottom. Arrange sliced bananas to cover the bottom. Set Aside.
To make the batter in a bowl stir together dry ingredients ( flour, whole wheat flour, baking powder, baking soda, cinnamon and salt), set aside.
In a large bowl cream 1/3 cup butter and ¾ cup sugar. Add eggs and vanilla and mix to combine. Add Greek yogurt and mashed bananas and mix to combine.
Turn your mixer on low and add dry ingredients mixture. Mix just to combine. Pour the batter over bananas, smooth the top and bake. Bake 30 minutes at 350 F, then reduce the heat to 325 F and bake for additional 25-35 minutes until the toothpick inserted in the center comes out completely clean.
Let it cool in the pan completely, or at least until the bottom is barely warm before inverting on the plate. Use a knife to loosen the edges from the pan, place a plate upside-down on top of the pan, then invert cake.
To make caramel glaze in a saucepan combine butter, brown sugar, heavy cream and salt and bring to boil over medium heat stirring constantly. Simmer for 2-3 minutes, remove from heat and let it sit for 1-2 minutes. Pour 2/3 of the glaze over the banana layer. Reserve leftovers of the caramel glaze for serving.
The glaze thickens as it cools and pale in colors!!!
When serving reheat the leftovers of the glaze and drizzle on each slice. Sprinkle with toasted chopped pecans if desired. You can also top each slice with a scoop of vanilla ice cream or whipped cream.