Preheat the oven to 425 degrees F and heat up an 8-inch cast-iron skillet.
You will need two medium bowls for the cornbread batter. In one bowl, whisk together the cornmeal, sea salt and baking soda. In the other bowl, whisk together the maple syrup, eggs, buttermilk and 4 tablespoons of the melted butter. Pour the wet ingredients into the dry and stir to combine¿it will look lumpy and kinda thick.
When the cast-iron skillet is very hot, add the remaining 1 tablespoon butter and then pour in the batter. Scatter the sausage over the top; add the jam in even, teaspoon-size dollops; sprinkle with the cheddar.
Bake on the middle rack until the center is set and the cheese is melted, 35 to 40 minutes. If you want the cornbread extra brown, put it under the broiler for 2 minutes.