Line a baking sheet with parchment paper or a silicone baking mat and set it near the stove. Measure out all ingredients before you start cooking, as the sugar changes stages quickly.
In a heavy-bottomed skillet or saucepan, add the raw peanuts, granulated sugar, water, and fine sea salt. If using ground cinnamon, nutmeg, allspice, or cayenne, add them now. Stir well so all the peanuts are evenly coated in the sugary water mixture.
2 cups raw peanuts, shelled, 1 cup granulated sugar, 1/2 cup water, 1/4 teaspoon fine sea salt, 1/2 teaspoon ground cinnamon, 1/8 teaspoon ground nutmeg or allspice, 1/8-1/4 teaspoon cayenne pepper
Place the pan over medium heat. Cook, stirring often and scraping the bottom and sides, until the sugar fully dissolves and forms a glossy, bubbling syrup coating the peanuts, about 4–6 minutes.
Continue cooking over medium heat, stirring every 20–30 seconds. The syrup will thicken, turn grainy, and eventually crystallize, becoming dry and sandy and clinging to the peanuts. This dull, sugary coating is expected and means you are at the correct stage.
Reduce the heat to medium-low. Keep stirring frequently so the peanuts and sugar do not scorch. As you stir, the sandy sugar will slowly begin to melt again and adhere more tightly, turning golden and slightly shiny. Continue until most of the sugar is melted and the peanuts are coated in a deep golden, glossy shell that smells nutty and caramel-like, about 5–8 minutes. Do not let the sugar turn dark brown or black.
Turn off the heat. If using vanilla extract and/or unsalted butter, quickly stir them into the hot peanuts until evenly distributed. The vanilla may steam briefly; this is normal. Work quickly, as the coating begins to harden as it cools.
1 teaspoon vanilla extract, 1 teaspoon unsalted butter
Immediately pour the hot sugared peanuts onto the prepared baking sheet. Use a spatula to spread them into a single layer, breaking up larger clumps while they are still warm. Allow the peanuts to cool completely at room temperature, 15–20 minutes, until the sugar shell is firm and crunchy.
Once fully cooled, break apart any remaining large clusters. Taste and, if desired, sprinkle with a small pinch of additional salt and toss gently. Serve immediately, or store in an airtight container once completely cool.
1/4 teaspoon fine sea salt