Preheat oven the 350° and line a baking sheet with parchment paper.
Combine the cornflour, cornmeal, and sea salt in a bowl, and stir well. Make a well in the center, and pour in the warm water and coconut oil. Stir well, then form into a tight ball. Set aside for 20 minutes, and work on the plantain filling.
Melt the coconut oil in a small sauce pot. Add the plantains to lightly fry, then reduce the heat and mash with a fork or potato masher. Add the sugar and cinnamon, and continue mashing until smooth.
To make the empanadas:
Break a ½ cup-sized piece off of the cornflour ball, and place it on top of one sheet of parchment paper. Cover with the second sheet, and use the rolling pin to roll the dough flat (about ¼ centimeter).
Remove the top sheet and cut the dough with your circular lid or saucer. Place the excess dough
back in the bowl with the dough ball.
Spoon 2 tablespoons of the plantain mash into the bottom half of the dough, but not too close to the edges. Use the parchment paper to fold the dough in half, and firmly press the edges to seal. Gently remove the empanada from the parchment paper, and place on the baking sheet.
Repeat with the remaining dough and plantain mash.