Preheat the oven to 350F degrees. Spray a 6 cavity doughnut pan with non-stick cooking spray. The recipe will make about 6-8 doughnuts, so you may need to bake them in 2 batches or prepare 2 doughnut pans if you have multiple.
In a large bowl whisk together the flour, baking powder, salt, cinnamon & nutmeg. Set aside. In a separate medium sized bowl whisk together the melted butter, milk, vanilla & sugar. Once combined, stir in the mashed/processed banana. Pour the wet ingredients into the flour mixture and stir together with a large wooden spoon or rubber spatula until well combined.
To fill the doughnut cavities, spoon the batter into a large ziplock bag then cut the bottom corner of the bag off. Use the bag to pipe the batter into the doughnut cavities. Then bake for 8-10 minutes until the doughnut feels slightly firm when gently touching with your finger.
Allow to cool in the pan for about 5-10 minutes then remove from the pan and continue cooling on a wire rack.
While the doughnuts are cooling, make the glaze. In a small bowl melt together the chocolate, butter, corn syrup and milk in short 30 second bursts stirring between each. Once the chocolate is entirely melted, whisk the batter until smooth. Dip the top of the tops of each doughnut in the chocolate and then sprinkle with walnuts.