How to Make It
Preheat the oven to 350° and line an 8-inch square baking pan with aluminum foil, allowing 2 inches of overhang on two sides. In a food processor, combine all of the ingredients and pulse until a dough forms, 1 to 2 minutes. Press the dough into the prepared pan in an even layer. Bake for about 25 minutes, until the shortbread is firm and the edges are golden; let cool completely.
In a small saucepan, melt the butter with the cream and salt over moderate heat; keep warm. In a medium saucepan, combine the sugar and water and cook over moderate heat, swirling without stirring, until a golden caramel forms, about 7 minutes. Carefully drizzle in the warm cream and cook over moderate heat, stirring constantly, until the temperature reaches 230° on a candy thermometer, 3 to 5 minutes. Immediately pour the caramel over the cooled shortbread and let cool completely, about 45 minutes.
Finely chop the chocolate and transfer it to a medium heatproof bowl. In a small saucepan, bring the cream just to a simmer. Immediately pour the hot cream over the chocolate and let stand until the chocolate starts to melt, 2 minutes; stir until thickened and smooth. Pour the ganache over the cooled caramel and spread it in an even layer with an offset spatula. Sprinkle with flaky sea salt and your choice of toppings and refrigerate until chilled, about 1 hour. Lift the square out of the pan using the long sides of the foil. Cut the shortbread into squares and serve slightly chilled.