Mix together sweet potatoes, salt, brown sugar, and vanilla. Spread into ramekins or serving bowls.: The moment you stir these components, a warm, inviting aroma will rise, signaling how the flavors are marrying. The mash should feel smooth and slightly fluffy, not gluey. I find using a wooden spoon or rubber spatula gives the best control for folding without overworking, which preserves a pleasant, soft texture. If the mixture seems dry, a teaspoon of water or a tiny dab of butter can smooth it, but resist adding too much liquid because the filling should hold shape under the sugar. A common mistake is under seasoning; taste and add a tiny extra pinch of kosher salt if needed.
Sprinkle sugar on top of the sweet potato mixture until well coated. Melt the sugar until it is lightly brown using a kitchen torch or the broiler of your oven. Watch carefully so that the sugar does not burn.: As you transfer the mash, notice its sheen and how it settles; it should hold a little peak but be soft enough to spread evenly. Using a spoon or offset spatula helps create a smooth surface, which is important so the topping caramelizes uniformly. The contrast between the warm mash and the cool dish can affect setting, so if you are making this ahead, allow chilled servings to come closer to room temperature before topping. Try not to compress the mash too much, as compacting can change the mouthfeel.
Sprinkle sugar on top of the sweet potato mixture until well coated: When you dust the surface with granulated sugar , pay attention to coverage; an even bed about a quarter to a half centimeter thick gives the best caramel layer. The dry sugar should glint under light, indicating an even spread. If the sugar clumps, break it up with a small spoon to avoid uneven browning. One mistake is adding too little sugar, which yields a thin, brittle crust instead of a satisfying crack.
Melt the sugar until it is lightly brown using a kitchen torch or the broiler of your oven: As the sugar melts, it will bubble, turn glossy, and then shift to a pale amber, releasing a rich caramel scent. With a torch, move in short sweeps to avoid scorching a spot; you should hear soft sizzling as the sugar liquefies. Under the broiler, position the rack close and watch constantly because browning happens fast. The visual cue to stop is a deep golden color, not dark brown, which means burnt. Overcooking produces a bitter note, so remove the dish at the first indication of evenly distributed color. A frequent pitfall is walking away while broiling, which risks burning the sugar and ruining the flavor.