Line 18 muffin cups with paper liners. Preheat the oven to 325°F.
Whisk the flour, baking soda, salt, baking powder, cinnamon, and nutmeg in a medium bowl to blend. Beat the sugar, oil, eggs, and vanilla in a large bowl to blend. Stir in the banana. Add the dry ingredients and stir just until blended.
Divide the batter among the prepared muffin cups. Bake the muffins on the middle rack until the tops are golden brown and a tester inserted into the center comes out with no crumbs attached, about 25 minutes. Transfer the muffins to a rack and cool slightly. The muffins may be eaten warm or cooled completely and frosted.
To frost the cupcakes: Using an electric mixer, beat the cream cheese and butter in a large bowl until light and fluffy. Beat in the mascarpone cheese and then beat in the honey. Spread the frosting over the muffins. Sprinkle with the walnuts.