Preheat the oven to 425°F along with the cast iron pan.
Melt half of the butter in the microwave for 30 seconds.
Add sugar and lemon zest to a small mixing bowl and mix together with a whisk or rub with your fingers to release the lemon oil.
In the same bowl, add flour and salt and mix together thoroughly with a whisk.
Transfer the flour mixture to a food processor, and add eggs, milk, vanilla and the melted butter.
Process until everything is thoroughly combined, about 30 seconds to 1 minute.
Add the rest of the butter to the hot cast iron pan, swirl around to coat the bottom and the side completely.
When all the butter has melted, add the batter to the pan and put it back into the oven.
Bake for 20 minutes without opening the oven door to allow the dutch baby to completely puff up.
Serve with fresh berries, lemon curd and powder sugar.