Preheat oven to 180°C | 350°F. Grease and flour a 8 x 8 inch square baking dish.
In a medium bowl, add wheat flour, all-pupose flour, oats, baking soda, salt and mix until well combined.
In a large mixing bowl, add the butter and sugar. Beat until light and fluffy. Add the eggs one by one and beat well.
Add the buttermilk, yogurt, vanilla essence, mashed bananas and blend until well combined.
Add the dry ingredients, pecans and fold until just incorporated. Pour batter into the prepared baking dish.
Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean. Remove and cool on a wire rack.
Microwave the nutella for 10-15 seconds and spread on the cake.