Heat your skillet over medium heat to melt the butter. Remove from heat for two or three minutes. Meanwhile, mash the bananas in a bowl. Set aside.
Add the brown sugar to the butter and stir until smooth. Add the eggs, one at a time, whisking after each until smooth. Add the vanilla, yogurt, buttermilk and stir to combine.
Whisk the flours, salt, baking powder and soda in a bowl. Stir into the skillet. Add the bananas and mix to combine.
Spoon half of the batter into the flour bowl. Cover the remaining batter with the cinnamon swirl filling. Smooth the remaining batter back over the top.
Pressing gently with your fingertips, evenly coat the top with the crumb filling.
Bake for 40 to 45 minutes, until a toothpick inserted in the center comes out clean. The cake will be a dark golden and the edges will be pulling from the side of the skillet. Allow to cool 5 minutes before running a pairing knife around the edges.
Serve warm from the oven with a scoop of vanilla ice cream or serve for breakfast the next morning with coffee. I really think the flavor gets better after a nice sit.