- With the rack in the middle position, preheat the oven to 350°F (180°C). Line a baking sheet with a silicone mat or parchment paper. Butter and line an 8-inch (20 cm) square dish with parchment paper, letting the paper hang over two sides.
- Scatter the peanuts on the baking sheet and roast for 12 minutes in the oven. Let cool.
- Meanwhile, in a pot over medium-high heat, bring all the ingredients, except the peanuts, to a boil. Continue boiling without stirring until the mixture reaches 241°F (116°C). Remove from the heat and let rest for 30 minutes without stirring.
- Add the peanuts. Using a spatula or wooden spoon, stir until the mixture thickens and lightens in colour, about 5 minutes. Quickly pour into the dish and smooth the surface. Let cool completely. Unmould and cut into 1-inch pieces. The fudge will keep for 2 weeks in an airtight container at room temperature.