Finely crush the Oreos with a food processor or blender. Stir crumbs together with 8 tablespoons melted butter until well combined. Press into the bottom and sides of a pie pan. Freeze the crust for 10 minutes until set
Combine the remaining 8 tablespoons of butter and brown sugar in a small saucepan. Cook over medium heat, whisking constantly until mixture begins to bubble. Continue cooking, whisking constantly, for 1 minute. Remove from heat. Slowly whisk in 1/4 cup heavy whipping cream until smooth. Cool caramel for about 15 minutes.
Pour the caramel over the Oreo crust, then return to the freezer for about 30-45 minutes until just chilled and set.
Chop the dark chocolate bar into fine pieces.
Place chopped chocolate in a glass bowl. In a saucepan, bring 1 cup heavy whipping cream to a simmer over medium-high heat. Pour the cream over the chocolate chips and let sit for 5 minutes, then whisk until completely smooth.
Pour the chocolate over the caramel and freeze for a final 30 minutes, until just chilled and set. OR refrigerate, covered, until ready to serve. Before serving, sprinkle the top with any flaky sea salt like kosher salt.
Serving Size 18-inch pie/tart