Preheat your oven to 350°F and position a rack in the center of the oven.
Grease a standard 8 1/2 by 4 1/2-inch loaf pan thoroughly with butter or nonstick spray.
In a large mixing bowl, mash the bananas and plantains together until the bananas are mostly smooth and the plantains are broken into small, grape-sized chunks.
Stir the sugar into the fruit mixture and let it sit for 15 minutes to allow the sugar to dissolve.
In a separate bowl, whisk together the flour, salt, cinnamon, baking soda, and mace.
Melt the butter in a small saucepan over low heat, then remove from the heat and stir in the cream cheese until smooth.
Whisk the vanilla and eggs into the butter mixture one at a time until the texture is smooth and creamy.
Combine the wet mixture with the fruit mixture, then fold in the dry ingredients until just incorporated, being careful not to overmix.
Fold in the toasted walnuts if you are using them, then pour the batter into your prepared loaf pan.
Bake for 80 to 85 minutes, or until the top is a deep golden brown and a tester inserted into the center comes out clean.
Cool the bread in the pan on a wire rack for 15 minutes before removing it from the pan to cool completely on the rack.