Place a rack in middle of oven; preheat to 400°. Wrap sweet potato tightly in foil and bake on a small rimmed baking sheet until a knife slides very easily into flesh, 35–45 minutes. Let cool.
Bring broth and cream to a simmer in a large saucepan over medium heat.
Meanwhile, season catfish fillets with Blackening Seasoning and heat cast iron pan with 2 tbsp oil.
Whisk grits into broth mixture, making sure there are no clumps. Add C’est Tout. Cook, still over medium heat, whisking often, until thick and creamy but with a little bite, 25–30 minutes. Add butter and smoked Gouda.
Unwrap sweet potato and peel. Transfer flesh to a medium bowl and mash until creamy and smooth (you should have about 2 cups). Add to grits and blend. Season as needed.
Place Catfish in hot skillet for 2-3 minutes on each side until done.
Transfer grits to a large shallow bowl; top with a pat of butter and season with more pepper. Top with Blackened Catfish.