Preheat the oven to 400°F. Line 2 baking sheets with parchment paper or silicone mats.
Peel the plantains and slice them as thinly and evenly as possible, using a mandoline if you have one.
Brush the parchment paper or silicone mats with melted coconut oil and spread the plantain slices out in a single layer.
Brush the tops of the slices with the remaining coconut oil and sprinkle with cinnamon sugar if you like.
Bake the plantain slices, one sheet at a time for 10 minutes then flip the slices over.
Bake on the second side for another 10 minutes, checking after 5 minutes to make sure they aren’t getting overdone. Depending on the thickness of your slices you may need a little less time on the second side, or a little more; just keep an eye on them and bake until they are browned and crispy.