Place the butter in a 12 inch wide cast iron pan with an oven safe handle. Place in the oven and preheat to 425°F.
Place the rest of the ingredients (except the blueberries if using!) into a blender, and blend on high until thoroughly blended. When the oven is preheated and the butter is melted and bubbly, (usually by the time your oven is up to temp your butter is ready!) carefully remove the cast iron pan from the oven (with oven mitts!) and pour the batter from the blender jar into the hot pan. Sprinkle the blueberries evenly and quickly over the batter, if using.
Return the pan to the oven and bake for 15-18 minutes or until the pancake puffs up and is golden around the edges. Remove from the oven, cut into 4 even wedges and serve hot with organic maple syrup!