Preheat oven to 375°F.
Place 2 tablespoons of butter into a cast iron or stone baking dish and set it in the oven for a few minutes to melt.
Add 1 cup of polenta to a blender or food processor and pulse for several seconds, until it reaches a flour-like consistency.
Sift the milled polenta together with all remaining dry ingredients. (Einkorn flour tends to be more lumpy than all-purpose flour, so sifting is recommended.)
In a separate bowl, whisk together the eggs, milk, remaining butter (melted), and sweetener until well combined.
Add the dry ingredients to the wet ingredients and mix until just combined.
Carefully pour the batter into the warm, buttered dish.
Bake for about 20 minutes, or until the top is golden.