Preheat your oven to 350°F (175°C). Grease or line a 9x5-inch loaf pan with parchment paper.
Blanch the callaloo in hot water, then allow it to cool completely. Squeeze out as much liquid as possible before finely blending with condensed milk or coconut milk.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, ground allspice, cinnamon, and nutmeg.
In a large bowl, mash the ripe plantains until mostly smooth. Whisk in the melted butter, brown sugar, granulated sugar, and eggs, and vanilla, until well combined.
Gradually fold the dry ingredients into the wet ingredients until just combined. Do not overmix.
Fold in the chopped callaloo and any optional mix-ins.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 15 minutes before transferring to a wire rack to cool completely.