Grease a 9x9-inch square baking pan and line with parchment paper so parchment paper overhangs edges by 1-inch.

Pulse cookies in a food processor until fine crumbs form.

Melt 1/2 cup butter and add to the cookie crumbs in the food processor; pulse until combined.

Press crumb mixture firmly and evenly into the bottom of the prepared baking pan using the flat bottom of a measuring cup or glass. Refrigerate for 30 minutes.

Combine 6 tablespoons butter, sweetened condensed milk, brown sugar, and salt in a saucepan. Cook over medium-high heat until boiling, stirring often.

Reduce heat to medium. Cook, stirring continuously, until a candy thermometer reads 230 to 235 degrees F or until thickened and darkened in color, 6 to 8 minutes. Pour caramel over the chilled shortbread base and spread evenly. Refrigerate for 2 hours.

Combine dark chocolate chips and remaining 2 tablespoons butter in a microwave-safe bowl.

Microwave chocolate on High until melted and smooth, 1 to 2 minutes, stirring every 20 seconds. Spread melted chocolate mixture evenly over the chilled caramel layer.

Sprinkle with coarse sea salt after standing for 10 minutes, if desired. Refrigerate until set, about 1 hour.

Remove from the pan, place on a cutting board, and let stand for 30 minutes.