Preheat the oven to 450° F.In a blender, combine the eggs, milk, flour, salt, vanilla, and 2 tablespoons melted butter. Blend 1 minute, until smooth. Make sure no large clumps of flour remain. Mix in the lemon zest, and then the jam, swirling it into the batter.Melt 2 tablespoons butter in a 10-12 inch cast iron skillet set over medium heat. Add the blueberries. Allow the butter to brown around the berries, about 3 minutes. Pour the batter into the hot skillet. Make sure all of the jam comes out. Place the skillet in the center of the oven and bake for 15 minutes or until the pancake is fully puffed and browned on top. DO NOT open the oven during the first 15 minutes of cooking or you might deflate your Dutch baby.As soon as the Dutch baby comes out of the oven, top it with pats of butter and serve drizzled with maple syrup. ENJOY Mom's favorite recipe (: