To make the crumb topping, place all ingredients in a small bowl and use your fingers to combine until the mixture resembles coarse wet sand. Evenly sprinkle the crumb topping over each muffin.
Beat all wet ingredients in separate large bowl, adding and beating the buttermilk last. Carefully fold in the flour in 3 batches just until combined.
Spray muffin pans with vegetable spray. Fill half full with the batter, put either one truffle in the center or 1 teaspoon Nutella. Add more batter on top to fill the muffin tin to just a little over ¾ full.
Bake for 16-18 minutes or until a toothpick inserted into the center comes out clean. Place the muffin pan on a cooling rack and allow it to cool for 5 minutes before removing the muffins.
Makes 12 muffins.
Recipe by The Daring Gourmet at https://www.daringgourmet.com/whole-wheat-banana-buttermilk-truffle-muffins/