Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
Make the dough: In a bowl combine 1 cup cornflour, ½ cup cornmeal, and ½ tsp sea salt. Stir to blend. Make a well in the center and pour in ½ cup warm water and 1 tbsp coconut oil. Stir until the mixture comes together, then form into a tight ball. Set the dough ball aside for 20 minutes.
Make the plantain filling: In a small saucepan, melt the remaining 1 tbsp coconut oil over medium heat. Add the 2 very ripe yellow plantains (chopped) and lightly fry until softened. Reduce the heat and mash the plantains with a fork or potato masher. Add 1 tsp coconut sugar and ½ tsp cinnamon and continue mashing until smooth. Remove from heat and let the filling cool slightly.
Prepare to roll: Place one sheet of parchment paper on your work surface. Break off a piece of dough about ½ cup in volume, and place it on the parchment. Cover the dough piece with a second sheet of parchment.
Roll and cut: Use a rolling pin to roll the dough flat between the two parchment sheets to about ¼ centimeter thickness. Remove the top sheet of parchment and cut dough rounds using a circular lid or saucer. Return any excess dough to the bowl with the dough ball to be re-rolled.
Fill and seal: Spoon 2 tablespoons of the plantain mash onto the bottom half of each dough round, leaving a clear margin around the edge. Fold the dough over to form a half-moon and firmly press the edges to seal. Optionally crimp the edges with a fork for a tighter seal. Gently transfer each sealed empanada to the prepared baking sheet, placing them seam-side down.
Repeat: Continue rolling, cutting, filling, and sealing with the remaining dough and filling.
Bake: Bake the empanadas on the prepared baking sheet for 25 minutes. Remove from the oven and let cool slightly before serving.