Line your 9x9 inch baking dish with parchment paper, leaving some overhang for easy removal.
Mix the graham cracker crumbs with melted butter until evenly coated and crumbly.
Press the buttery crumbs firmly into the prepared dish to form a compact crust. Chill in the fridge for 15 minutes to set.
In a saucepan over medium-low heat, warm the sweetened condensed milk and caramel sauce, stirring constantly until the mixture is smooth, thick, and bubbling gently, about 5-7 minutes.
Pour the warm caramel over the chilled crust, spreading it evenly with a spatula. Return to the fridge and chill for at least 2 hours or until firm.
While the caramel sets, melt the dark chocolate in a microwave in short bursts, stirring until glossy and smooth.
Pour the melted chocolate over the set caramel layer, spreading it into an even, glossy top. Sprinkle with sea salt flakes for flavor and texture.
Place the entire dish back in the fridge and chill for about 30 minutes until the chocolate is firm and shiny.
Once set, lift the bars out of the dish using the parchment overhang and place on a cutting board. Use a sharp knife to cut into 16 squares, wiping the blade between cuts for clean edges.
Serve immediately or store in an airtight container in the fridge for up to 5 days. Enjoy that rich, gooey, and crunchy texture with every bite!