Place the pork belly in a large sealable plastic bag. Mix 3 cups of the water with the salt and sugar, and pour it into the bag. Seal tightly and let marinate 12 to 24 hours in the refrigerator.
When ready to cook the pork belly, preheat the oven to 300 degrees Fahrenheit with the rack in the middle of the oven.
Remove the pork belly from the brine and place in a small baking dish. Add the chicken broth and remaining 1/2 cup of water. Cover tightly with aluminum foil and roast for 2 to 2 1/2 hours until pork belly is fork tender. Remove the aluminum foil, increase heat to 450 and roast for about 20 minutes until the top has crisped and browned and the inner fat has mostly melted. Remove from oven and let cool. Once cool enough to handle, slice into 1-inch pieces.
To a saucepan set over medium-low heat, add the butter. Once it has melted and is bubbling, add the garlic and cook until fragrant, about 1 minute. Add the soy sauce, sherry vinegar, agave syrup and orange juice, and stir to mix well. Poke a few holes in the habanero with a pairing knife and add it to the sauce whole. It will release the flavor of the pepper without making the sauce super spicy. Bring to a simmer and let cook about 8 to 10 minutes until the sauce thickens and coats the back of a spoon.
In a large cast iron pan set over medium-high heat, heat the peanut oil. Add the brussel sprouts, season with salt and pepper to taste, and cook for about 12-15 minutes until crispy and browned on the outside and tender inside.
Plate the brussels sprouts on a large platter and top with the pork belly, drizzle with the sauce, and garnish with freshly sliced scallions.