Preheat the oven to 350°F (175°C). Place a cast iron pan into the oven to heat while you put the batter together.
Whisk together the cornmeal, masa harina, flour, baking soda, baking powder, and salt in a medium-sized mixing bowl.
In a small bowl, using the tines of a fork or a small whisk, beat together the egg replacer powder and maple syrup for about 60 seconds until smooth and a little foamy.
Using a rubber spatula, add the maple/egg replacer slurry into the bowl with the dry ingredients. Add the milk, cider vinegar, and olive oil, and mix together until smooth.
If using any mix-ins, stir in the corn and scallions last. If you are going to be serving the cornbread with BBQ dishes, you can also stir in some minced chilies.
Remove the hot skillet from the oven, and spread the margarine around the bottom and sides of the pan.
Using a rubber spatula, add the cornbread batter, and spread it quickly around into an even layer. Place the cornbread back into the oven and bake for 50-55 minutes until a toothpick can be removed from the center without having wet batter adhere to it, and the sides of the cornbread should appear lightly browned and pulling away from the edges of the pan.
Remove the cornbread from the oven, and just before serving, place a pat of vegan butter in the center and garnish with fresh herbs.