Spread chopped 1/4 cup walnuts, on an ungreased sheet pan and bake at 350°F degrees for 6-8 minutes. Cool and store if not using right away.
In a large mixing bowl or the bowl of a stand mixer, cream 3 Tablespoons of the butter with 1 cup of the sugar until light.
Add in the 2 medium overripe bananas, 1/4 cup sour cream and 1 large egg
Sift or whisk the flour with the 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 1/2 teaspoon cinnamon, and the pinch ground nutmeg . Add to the wet ingredients; mix only until combined.
Spread batter into a greased 8" round cake pan.
Melt the remaining 2 Tablespoons of butter in a small mixing bowl. Add in the remaining 2 Tablespoons of sugar, 2 Tablespoons of flour, 1 teaspoon of cinnamon and the toasted walnuts. Stir until all combined (will be thick).
Dollop the cinnamon streusel nut mixture on top of the cake batter; use a knife to swirl the cinnamon mixture into the cake batter.
Bake the cake at 350℉ for 20-22 minutes, or until a toothpick inserted in the center of the cake comes out clean.
Once the cake has cooled completely, combine the ingredients for the glaze (1 cup powdered sugar, 1 1/2 teaspoons ground cinnamon, 2-3 teaspoons water) adding enough water to make it a drizzling consistency. Drizzle on the cooled cake before serving.