Line a baking sheet with parchment paper and set aside.
Place the butter in a microwave-safe bowl and microwave in 15-20 second bursts until just melted. It should be warm and slightly bubbly.
Add the brown sugar to the melted butter and stir until combined and smooth. The mixture will be glossy and start to thicken slightly.
Pour in the sweetened condensed milk and vanilla extract, then stir steadily until the mixture is smooth, shiny, and begins to bubble gently.
Transfer the mixture to a saucepan and cook over low heat, stirring constantly. Cook for about 5-7 minutes until it thickens, turns a deep caramel color, and smells rich and buttery.
If using, fold in chopped nuts and chocolate chunks immediately after removing from heat, mixing well to distribute evenly.
Using a small cookie scoop or teaspoon, drop dollops of the caramel onto the prepared parchment paper. Flatten each slightly with the back of a spoon or spatula for even bites.
Sprinkle a pinch of flaky sea salt on top of each bite and gently press it into the surface for a salty crunch.
Let the bites cool at room temperature for about 15 minutes, then transfer to the fridge for at least 30 minutes until firm and chewy.
Once set, remove from the fridge, let sit at room temperature for 5 minutes, then enjoy these sticky, sweet treats!
