Line your small baking dish with parchment paper, leaving overhang for easy removal.
In a mixing bowl, combine the sweetened condensed milk, softened butter, and brown sugar. Stir until the mixture is smooth, glossy, and well combined, about 2 minutes.
Pour the mixture into your prepared dish and use a spatula to spread it evenly across the surface, smoothing the top for a flat, uniform layer.
Refrigerate the dish for at least 30 minutes until the caramel mixture is firm enough to handle without sticking excessively.
Once chilled, use a sharp knife to cut into small squares or rectangles, about 2 cm each, working quickly to maintain shape as the mixture softens at room temperature.
Meanwhile, melt the dark chocolate in a microwave-safe bowl in short 20-second bursts, stirring between each until smooth and glossy.
Using a fork, dip each caramel piece into the melted chocolate, turning to coat evenly, then gently tap off any excess chocolate.
Place each coated piece back onto a parchment-lined tray and sprinkle with a tiny pinch of sea salt if desired.
Refrigerate the caramel bites for an additional 15-20 minutes until the chocolate is set and shiny.
Remove the bites from the fridge, let sit at room temperature for 5 minutes to soften slightly, then enjoy these rich, chewy treats.
