Preheat oven to 350°F. Place tart pan on a parchment lined baking sheet and set aside.

In a large bowl, combine all crust ingredients. Mix thoroughly, then press mixture evenly into the bottom and sides of a 13.5" by 4" rectangular tart pan, using a measuring cup with a flat bottom to help flatten and compact the crust. Rest in the fridge for 15 minutes.

Bake crust for 10-13 minutes, remove from oven and place on cooling rack, allow to cool completely.

To make the caramel filling, in a large saucepan with a light-colored interior, combine the sugar, water, and corn syrup, then turn heat to medium-high.

Cook undisturbed (avoid stirring) until the sugar takes on an amber or pale copper color, about 10 minutes. If browning and cooking appears uneven, give pan a slight tilt or swirl.

Remove pan from heat and carefully whisk in butter until melted, it will steam and boil up, so be careful. Add the heavy cream, creme fraiche or sour cream and a couple pinches of salt, stir until smooth.

Pour caramel into prepared crust and smooth into an even layer. Let caramel set in the fridge until cool and firm, about an hour.

Prepare chocolate ganache by placing chopped chocolate (or chocolate chips) in a heatproof bowl and bringing cream to a simmer in a clean saucepan. Pour hot cream over chocolate and let sit undisturbed for 2 minutes, then whisk until smooth.

Pour ganache over cooled caramel layer, spreading evenly with a spatula or butter knife. Let set in the fridge, another hour, or until firm to the touch.

When serving, unmold tart from pan, sprinkle the top of the tart with flaky sea salt. Use a sharp knife dipped in hot water to cut, slices can become messy within a few minutes, so try to plate slices quickly.

Leftovers can be stored in the refrigerator for up to 5 days.