Preheat oven to 425 degrees. Place 4 mini cast iron pans in the oven to heat.
Blend eggs, milk, flour, cornstarch, sea salt, and zest of ½ a lemon in a blender and allow to rest.
When oven is nearing temperature, divide the butter between the skillets and allow to begin browning. Then, when oven is hot and butter smells nutty and is lightly browned, pour in batter, dividing between the 4 skillets.
Bake for 12-15 minutes on the center rack, and then in the final 2-3 minutes dust each dutch baby with powdered sugar and move up to the top ⅓ of the oven. When the sugar is lightly toasted and the dutch baby pancake is puffed and browned it is ready. Pull from the oven and serve with a squeeze of lemon, fresh berries, and maple syrup or honey, if desired.