Whisk together eggs, milk, sugar, salt, and vanilla extract. In a separate bowl whisk together flour and baking powder until blended. Dump the dry ingredients into the wet and whisk until homogenous. Let stand at least 10 minutes.
Heat an 8 or 10-inch cast iron skillet in the oven with the butter.
Pour the batter into the cast iron pan and bake until the pancake is golden brown, puffed up and set in the center, about 15 minutes.
To make the mixed berry syrup: Heat the berries in a small saucepan with the maple syrup until mixture thickens, but you can still see the berries. Serve warm over a wedge of pancake, dusted with powdered sugar.