For The Rolls

1. Add 1 teaspoon white granulated sugar to milk and sprinkle yeast on top. Let sit for about 10 minutes or until frothy and fragrant. 
2. To the bowl of a stand mixer fitted with dough hook, add yeast mixture, the rest of the granulated sugar, and egg yolk. Mix on low until combined, then add the bananas and mix them in as well.
3. Add melted butter and salt to bowl, mix for 1 minute, then add the flour slowly in several additions while mixing on low. Check the dough after about 3 1/2 cups of flour have been added. It should be fairly sticky but not wet. If too wet, add more flour.
4. Mix on medium-high speed for 10 minutes. While the dough is mixing, oil a large mixing bowl. Turn dough out into oiled bowl and turn over to coat.
5. Cover loosely with warm damp dish towel and set in a warm place for about 90 minutes, or until doubled in size.

Filling And Assembly

1. Grease a large rectangular baking dish. In a bowl, stir together brown sugar, cinnamon, nutmeg, and toasted walnuts.
2. Punch down dough and turn out onto a floured work surface. Use a floured rolling pin to roll dough out into a rectangle approximately 15 x 17 inches. Spread with 6 tablespoons unsalted butter.
3. Sprinkle dough with filling mixture, making sure to cover all the way out to the edges. Top with thinly-sliced banana.
4. Starting on the long edge, tightly roll up the dough into a log. Use your hands to gently stretch and squish the log until it's relatively the same diameter along its entire length. Use a sharp, serrated edge knife to cut into 15-18 equal slices and set into prepared pan. 
5. Cover loosely with plastic wrap and let rise in a warm place for an additional 30 minutes or until the rolls look puffy. Preheat oven to 350F.
6. Bake for 25-30 minutes. You want the rolls to look slightly golden but not be overbaked and dried out. Let cool in pan a few minutes while preparing frosting.

Icing and Serving

1. Use a hand mixer on low to combine all icing ingredients, adding the powdered sugar in last. Spread over warm cinnamon rolls - as little or as much as you want - and serve immediately.