Preheat the oven to 425°F. Slice the butter into cubes and place in a large cast iron skillet or Dutch oven (no lid). Place the skillet in the oven and let the butter melt.
Meanwhile, in the bowl of a blender or food processor, blend the eggs until foamy. Add the buttermilk, flour, sugar, salt, and vanilla and blend until combined.
When the butter is fully melted, swirl it in the pan to evenly coat the surface. Be sure to use oven mitts as the handle(s) will be hot. Pour the batter in the skillet and scatter the berries. Return the skillet to the oven and bake for about 25 minutes or until the pancake has puffed and is golden brown.
Remove from the oven and cut into wedges. Sprinkle with powdered sugar and serve immediately.
The batter can also be portioned into smaller pancakes if you would like to divide into individual servings.