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.
Whisk the eggs in a large bowl, add the cooked chorizo and plantains; season with salt and pepper. Gently stir to incorporate.
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.