Preheat the oven to 350 degrees F.
Cut the bread into cubes and spread out on a baking pan. Bake for 5 to 7 minutes until golden and toasted. Set aside.
Coat a nonstick skillet with the oil and place over medium-high heat. Add the chorizo and cook until completely browned, breaking it up with a wooden spoon, about 5 minutes. Transfer to a paper towel-lined plate and set aside.
Add the plantains to skillet and cook until browned, 3 to 4 minutes, stirring often. Transfer the plantains to a paper towel-lined plate.
Melt the butter in the skillet over medium heat. Add the egg mixture, tilting the pan so that the eggs spread out into a thin layer. Once you see the eggs on top cover the skillet and cook until completely set and cooked through, about 10 minutes. Carefully slide the frittata onto a serving dish, slice into wedges, and serve with bread cubes sprinkled on top.