Preheat the oven to 425°F. Place a 10-inch cast iron skillet in the oven while it preheats.
Grate the zest of the lemon to equal 1 teaspoon. Squeeze the juice from the lemon to equal 1 tablespoon. Combine the strawberries, raspberries, blueberries, lemon juice, and 1 tablespoon of the granulated sugar in a medium bowl. Toss to coat the berries well, and set aside. (The berries will release their juices as they sit in mixture, creating a sweet, syrupy sauce.)
Combine the eggs, milk, flour, lemon zest, remaining 1 tablespoon granulated sugar, salt, and vanilla in a blender. Blend at high speed for 1 minute. Set aside.
Remove the hot skillet from the oven and add the butter, swirling the skillet to coat the bottom of the pan evenly with melted butter.
Add the batter to the hot skillet. Transfer the skillet to the oven and bake for 15 to 17 minutes or until the Dutch baby puffs up and becomes golden brown.
Remove from oven and let cool slightly. To serve, spoon the berry mixture over the top of the pancake and dust with powdered sugar. Serve with sweetened whipped cream.