Arrange a rack in the upper third of the oven and heat the oven to 375°F. Place a 10-inch cast iron skillet in the oven while it heats.

Place 1 ¼ cups all-purpose flour, 2 tablespoons granulated sugar, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon kosher salt together in a large bowl and whisk to combine.

Place 2 tablespoons of the unsalted butter in a medium microwave-safe bowl. Microwave in 15-second bursts until melted, 30 to 40 seconds total. (Alternatively, melt on the stovetop and transfer to a medium bowl.)

Add 1 ¼ cups buttermilk to the melted butter. Separate 2 large eggs, adding the yolks to the butter mixture and the whites to the now-empty buttermilk measuring cup. Whisk the egg yolk mixture until combined.

Pour the buttermilk mixture into the flour mixture and stir with a flexible spatula until most of the flour is moistened. Add the egg whites and fold in until incorporated and no streaks remain; do not overmix. Let the batter rest for 10 minutes. Meanwhile, trim and cut ½ cup fresh strawberries into ½-inch pieces. Cut the remaining 2 tablespoons unsalted butter into 4 pieces.

Take the preheated skillet out of the oven. Add the butter and swirl the pan until the butter melts and coats the bottom and sides. Transfer the pancake batter to the pan and spread into an even layer. Sprinkle the strawberries, ½ cup fresh blueberries, and ½ cup fresh raspberries evenly over the pancake batter.

Bake the pancake until puffed and golden-brown, 22 to 25 minutes. Cut into wedges and serve with more butter, maple syrup or powdered sugar, and more berries.