First, make the topping. Preheat the oven to 350°F. Lightly grease 4 oven-safe (8-ounce) ramekins. Slice the orange into 8-10 slices, about ¼-inch thick each. With a sharp knife, carefully remove the outer peel from each slice (do this after slicing to keep each slice better intact). Sprinkle 1½ teaspoons of brown sugar into the bottom of each ramekin. Place 2 orange slices into each ramekin on top of the sugar. Pour ½ tablespoon of melted butter into each ramekin. Bake for 5 minutes, until the butter starts to bubble. Carefully remove the ramekins.
Next, make the cakes. In a medium bowl, whisk together flour, baking soda, and salt.
In the bowl of a stand mixer, add softened butter and sugar. Beat on medium-high for 2-3 minutes. Add egg and vanilla. Stop to scrape down the sides.
Add half the flour mixture to the bowl, then the buttermilk, then the rest of the flour mixture and mix until just blended. Pour batter into hot ramekins, only filling ⅔ full. (Note: You may have extra batter for an additional ramekin.) Bake for 20 minutes, or until a toothpick inserted in the center comes out clean. Let cool for 10 minutes, run a knife around the edges, then invert onto a serving plate. Serve with ice cream or coconut whipped cream.