Preheat oven to 425 degrees F. While the oven is preheating, place the butter in a cast iron skillet and place in the oven to melt.
Place the eggs, milk, flour, vanilla, and salt in a blender and blend until completely smooth, about 2 minutes. The longer you blend, the fluffier your German pancake will be.
Pour the batter over the melted butter in the cast iron skillet. Arrange chopped rhubarb and sliced strawberries on top.
Bake in preheated oven until puffy and golden brown, about 20-25 minutes.
Serve hot with maple syrup and whipped cream or vanilla ice cream.