Preheat oven to 350 degrees and spray an 8x8 baking dish with non-stick spray, set aside.
In a small bowl combine vinegar with milk, set aside.
In a medium sized bowl combine flour, baking powder, baking soda, cinnamon, salt and nutmeg. Set aside.
Mash bananas into a larger bowl. Add in yogurt, sugars, egg, and vanilla. Mix well.
Now, add 1/2 of the flour mixture to the banana mixture, mixing well. Then 1/2 of the buttermilk, mixing well. Repeat.
Pour batter into prepared baking pan and bake for 35 minutes until golden brown and a toothpick comes out clean.
While the cake in baking you can make the greek yogurt frosting. Simple combine the greek yogurt, powdered sugar and coconut flour into a bowl and whisk until well combined and is fluffy. Allow to sit in the fridge for ~1 hour while the cake bakes and cools.
Once the cake is fully cooled frost the cake with your frosting.
Enjoy!