½tspcoarse Kosher salt(if using table salt, use half the amount)
½cupcold unsalted butter,diced
½cuplight corn syrup
½cupdark brown sugar,lightly packed
½cupsweetened condensed milk
¼tspcoarse Kosher salt(if using table salt, use half the amount)
4ozheavy whipping cream
4ozgood-quality dark or semi-sweet chocolate,chopped
¼tspflaky finishing salt,such as Maldon
Preheat the oven to 350 F. Line an 8x8 inch square baking dish with two sheets of parchment paper, so that they completely line the bottom and sides of the pan. Crease the paper at the top edges of the pan to help keep it in place.
In a bowl, combine the flour, sugar, salt and vanilla. Add the butter and use a pastry cutter to cut the butter into the flour mixture until well incorporated, and the crumbs are moist and crumbly.Dump the dough crumbs into the prepared pan. Spread out evenly in the pan, then use your hands to press the crumbs down firmly against the bottom of the pan.
Bake the crust on the center oven rack for 20 minutes, until pale golden brown.Set the pan on a cooling rack and cool for 20 minutes.(Don't start making the caramel until after you've cooled the shortbread, since the caramel cooks quickly.)
In a large microwave-safe bowl (I recommend a glass Pyrex bowl with a 2.5 quart capacity), whisk together the melted butter, corn syrup, granulated sugar, brown sugar, sweetened condensed milk and salt until smooth.
Place the bowl in your microwave, uncovered. Microwave on high for 6 minutes, then stir and check the temperature with a digital instant read thermometer. The temperature of the caramel needs to reach between 238-242 degrees F.If the caramel isn't hot enough, continue to microwave on high in 15-second increments, checking the temperature each time until it reaches the correct temperature.My microwave is 1,000 watts, and it usually takes around 6 1/2 minutes.Note that if your microwave is a higher or lower wattage, it may take more or less time. Also, the temperature of your ingredients when you start can also affect how long it takes, but getting the caramel to the right temperature is the only thing that matters.
Once the caramel reaches 238 degrees, take it out of the microwave, using oven mitts if the bowl is hot. Immediately stir in the vanilla until smooth; the caramel will bubble and hiss a little.
Immediately pour the hot caramel over the cooled crust.Let the bars sit at room temperature to cool for 1 hour, then refrigerate until both the crust and the caramel are completely cooled and firm, about 1 - 1 1/2 hours.
Set up a double boiler (a pan of simmering water with a heatproof bowl sitting on top of the pan - the bottom of the bowl should not touch the water).
Measure the chocolate and cream into the bowl. Let the mixture warm until the chocolate melts, then stir the ganache with a spatula until smooth and shiny.
Immediately pour the ganache over the cooled caramel, and smooth out the ganache.Let the bars sit at room temperature to let the ganache set for about 30 minutes, then refrigerate until the ganache is firm.
Cutting the Bars
These bars are best eaten at room temperature when the caramel is soft and luscious. But when you cut the bars, you want them to be chilled and firm enough to cut cleanly, although not rock hard from the fridge.Once the ganache has set, take the bars out of the fridge. Lift the bars out of the pan by grabbing onto the parchment paper. Pull the paper away from the sides of the bars, and then slide the whole slab of bars off the paper and onto a cutting board.
Press against the caramel around the edges. If it's soft enough that you can make an impression, then they're ready to be cut. You don't want to let them warm up enough that the caramel gets soft and gooey, or they will be hard to cut cleanly, and you'll need to chill them again before cutting.Use a large, sharp chef's knife to cut straight down, cutting the whole slab in half. Slide the knife out sideways, not back up through the bars. Then cut each half into 8 squares, for a total of 16 squares.To keep the bars looking neat, wipe the knife clean in between cuts.
Before serving, sprinkle the bars with the flaky salt.
These bars should be stored in an airtight container, in a cool dry place, for up to 3 days.