Preheat the Oven: Preheat your oven to 450°F (230°C). This high temperature is crucial for creating the dramatic puff of the Dutch baby pancake. Make sure the oven is fully heated before you put the batter in.

Prepare the Batter: In a blender, combine the eggs, whole milk, flour, salt, vanilla extract, and 2 tablespoons of melted butter. Blend for about 1 minute until the batter is smooth and no flour clumps remain. The blender creates a silky-smooth texture that's difficult to achieve by hand whisking. Add the lemon zest and swirl in the blueberry jam to create a beautiful marbled effect throughout the batter.

Cook the Blueberries: In a 10-12 inch cast iron skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the blueberries to the skillet and allow the butter to brown slightly around the berries, about 3 minutes. This process caramelizes the blueberries slightly and infuses the butter with their flavor, creating a delicious foundation for your Dutch baby.

Bake the Dutch Baby: Once the blueberries are cooked, pour the prepared batter into the hot skillet, ensuring any remaining jam is added. Transfer the skillet to the oven and bake for 15 minutes, or until the Dutch baby is puffed and golden on top. Avoid opening the oven during baking to prevent deflating the pancake. The dramatic rise happens due to the steam created by the hot skillet and high oven temperature.

Serve: Once the Dutch baby is done baking, remove it from the oven and top it with pats of butter. Drizzle with maple syrup and serve immediately. The Dutch baby will begin to deflate as soon as it comes out of the oven, so timing is everything for presentation.