Preheat the oven to 425°F.
Melt the butter in a 9-10-inch cast-iron skillet. When butter is melted, pour into small cup or ramekin and reserve. Hold butter in a warm place so it does not begin to set. Set buttered skillet aside.
Combine the cornmeal, flour, sugar, baking powder, salt, ground fennel seed, and baking soda in a large bowl and whisk to combine.
Add the buttermilk, eggs, and melted butter and stir until just combined. Do not over-mix. The batter should be lumpy.
Pour the batter into the buttered skillet and sprinkle the top with a little salt and the whole fennel seeds.
Bake until the top is golden brown and a sharp knife or cake tester inserted in the center comes out clean, about 25 minutes.
Remove from the oven, turn the cornbread out of the skillet, slice into wedges, and serve warm.