Preheat the oven to 350°F and line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides.
Beat the butter and sugar for 2 minutes until creamy. Add the flour and salt and mix on low until a soft, crumbly dough forms, then press it into an even layer in the pan and prick all over with a fork.
Bake for 22-26 minutes until golden brown at the edges and lightly golden in the center. Let it cool while you make the caramel.
In a medium saucepan over medium-low heat, melt the butter, brown sugar and corn syrup together, stirring until the sugar dissolves. Stir in the condensed coconut milk and salt.Bring to a gentle boil and cook, stirring constantly, for 6-8 minutes until the caramel is thick, deep golden and pulls away from the bottom of the pan. Remove from the heat and stir in the vanilla.
Pour the caramel over the shortbread and spread it into an even layer. Refrigerate for 1 hour until firm.
Melt the chocolate with the coconut oil in the microwave in 20-second bursts, stirring until smooth. Pour it over the caramel, spread evenly and sprinkle with flaky sea salt.
Chill for 30 minutes until the chocolate is set. Lift the slab out using the parchment, let it sit at room temperature for 10 minutes so the chocolate doesn't crack, and cut into 16 squares.