Let it melt completely, then continue cooking for 5-7 minutes, stirring often, as it begins to foam. You'll notice golden specks forming on the bottom and a toasted, nutty aroma, which means the butter is browned. Immediately remove it from the heat.
Add the coconut sugar and ¼ teaspoon of salt and stir for 2 minutes.
Place the sweet potato cubes into a large bowl, pour the brown butter mixture over them, and stir to coat.
Transfer the potatoes to a baking dish, cover them tightly with foil, and bake for 45 minutes.
Remove the foil and bake for another 20-30 minutes, until the sweet potatoes are fork-tender and the brown butter sauce is bubbling and slightly thickened.
Garnish with flaky salt and chives if desired, and serve.