In a medium saucepan, warm the sugar and butter over low-to-medium heat. Give the pan a shake to evenly coat the bottom of the pan with the sugar and keep a close eye on the pan. You’ll notice that the sugar around the sides of the pan will begin to melt first. As the sugar begin to melt, give the pan a swirl or use a rubber spatula to pull the caramel into the middle of the pan, covering the unmelted sugar and moving the unmelted sugar to the edges. As more sugar begins to caramelize, give it a stir until all of the sugar has melted and let it cook for a couple of minutes until you’re left with a deep amber color. Once the caramel reaches a deep amber, remove from the heat. Quickly stir in one squeeze of lemon juice (be careful, the caramel may bubble a bit). Immediately pour the caramel mix over the toasted nuts. Leave to set at room temperature or place in the fridge or freezer to hurry up the cooling process.