Preheat oven to 375F. Melt the butter and set aside to cool slightly.
In a medium-large mixing bowl whisk together the flour, sugar, baking powder, soda and salt. Make a well in the center. In a separate medium mixing bowl combine the buttermilk, vanilla, yogurt and eggs. Whisk until blended. Add the melted butter and whisk to combine. Pour the egg mixture into the dry mixture and gently fold together with a spatula until only a few streaks of flour remain. The batter will be a little lumpy. Add 1 cup of the fresh blueberries to the batter. Set the batter aside to rest for 10-minutes before baking.
While the batter is resting, lightly grease a 9-inch cast iron skillet and place in the oven to preheat. (see notes)
Remove the hot skillet from the oven and pour in the pancake batter. Quickly top with the remaining 1/2 cup of blueberries and the sliced strawberries. Sprinkle the coarse sugar on top and bake immediately for 25 minutes or until a toothpick inserted in the center comes out clean.