Preheat the oven to 425°F / 220°C.
Place the butter in a 10-inch cast-iron skillet and set it in the oven to melt while it preheats.
In a medium bowl, whisk the eggs until frothy.
Add the water, heavy cream, coconut flour, granular erythritol, vanilla, and salt.
Stir until the batter is smooth and well-blended.
Carefully remove the hot skillet from the oven. Swirl the melted butter to coat the bottom and sides, then pour the batter into the center of the skillet.
Bake for 18 to 20 minutes, until the pancake is dramatically puffed and golden brown.
Slice into quarters and serve immediately, garnished with fresh blueberries and a dusting of confectioners'-style erythritol, if desired.
Don't have a cast-iron skillet? Any 10-inch oven-safe pan or even a 9-inch pie plate will work.
