Preheat oven to 425 ℉. Place a 12-inch cast-iron skillet into the oven while it's preheating. In a medium size mixing bowl, add the sugar and lemon zest. Rub together with your fingers until the sugar is moist and fragrant. Add the remaining Dutch baby ingredients and whisk very well until no lumps of flour remain.
Remove the hot skillet from preheated oven and add the butter. Swirl the pan around to coat with melted butter. Pour in the batter and bake for 10 minutes. Reduce oven temperature to 400 ℉ and bake for an additional 10 minutes. The sides should be a medium golden brown while the center is fully cooked (there will be pools of melted butter on top).
While the dutch baby is cooking, make the filling. Use a fork to mix together all of the filling ingredients until well combined. Store in the fridge until the Dutch baby is ready.
Remove dutch baby from oven and immediately dust with powdered sugar and cut into slices. Serve with the lemon ricotta topping and enjoy.