In a medium mixing bowl, whisk together the flour, baking powder, baking soda, and cinnamon until thoroughly combined.
Peel and pit the mangos. Puree 1 1/2 mangos and dice up the remaining half.
In a stand mixer or with a hand mixer, cream together butter and sugar. Add the mango puree, egg, lime juice and vanilla, and mix well.
Stir the dry mixture into the mango mixture until just moistened. Fold in the diced mango and walnuts.
Spray a 9x5x3" pan with baking spray or rub it down with coconut oil, and pour the batter into the loaf pan. Garnish with thin mango slices if desired on top.
Bake for 45-48 minutes, until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes and then remove to a cooling rack, allowing the bread to cool for at least one hour before serving.