Preheat the oven to 425°F. Grease an 8" cast iron skillet with butter.
In a large bowl, whisk together the flour, cornmeal, sugar, baking powder, and salt. Place the cranberries in a separate bowl, sprinkle 1/4 cup of the flour mixture over them, tossing to coat, and set aside.
In a large measuring cup, whisk together the eggs and buttermilk. Make a well in the center of the dry ingredients and add the egg mixture, stirring to combine. Stir in the melted butter and the cranberries. Pour the batter into the greased skillet and smooth the top.
Bake the cornbread for 20 to 25 minutes, or until the top is golden and a tester inserted in the center comes out clean. Remove from the oven and serve hot with whipped molasses butter.
Whipped molasses butter:
Line a small bowl with enough plastic wrap to hang over the sides. Whip together 1/2 cup soft unsalted butter, 2 tablespoons molasses, and 1/4 teaspoon salt until combined. Transfer the mixture to the lined bowl, fold the excess wrap over the top, and refrigerate until set, about 1 hour.