Millionaire’s Shortbread

1. Place 2 sticks unsalted butter in the bowl of a stand mixer (or large bowl if using a hand mixer). Let sit at room temperature until softened. Meanwhile, line a 9-inch square baking pan with 2 sheets of parchment paper: Place them perpendicular to each other in the pan and make them long enough to they hang over the sides to create a sling. Lightly coat the parchment with cooking spray. Place 1 2/3 cups all-purpose flour and 1/4 cup potato starch or cornstarch in a medium bowl and whisk to combine.

2. Add 3/4 cup granulated sugar, 1 tablespoon vanilla extract, 1 teaspoon kosher salt, 1/2 teaspoon baking powder to the bowl of butter. Beat with the paddle attachment on medium speed until well creamed, light, and fluffy, about 5 minutes.

3. Add the flour mixture. Beat on low speed until combined and a thick dough similar to cake batter forms, 1 to 2 minutes.

4. Transfer the dough to the pan and spread into an even layer with an offset spatula. Cover with plastic wrap and refrigerate for at least 1 hour and up to 2 days.

5. Arrange a rack in the middle of the oven and heat the oven to 350°F. Uncover the dough and pierce it all over with a fork. Bake until golden brown, 20 to 35 minutes. Let cool in the pan until room temperature, about 1 hour. If the dough puffs up considerably, very lightly press into the dough with parchment paper or an offset spatula. Meanwhile, make the caramel.

6. Juice 1/2 medium lemon until you have 2 teaspoons. Place 1 (14-ounce) can sweetened condensed milk, 3/4 cup packed dark brown sugar, 3/4 cup maple syrup, 1/2 cup heavy cream, 1 stick unsalted butter, and 1 teaspoon kosher salt in a large, heavy-bottomed saucepan. Bring to a simmer over medium heat, whisking constantly. Continue to cook, whisking constantly, until it reaches 235ºF, 15 to 20 minutes.

7. Remove the pan from the heat. Add the lemon juice and quickly whisk to combine. Pour the caramel over the shortbread and spread with an offset spatula into an even layer, pushing the caramel into all the corners. Tap the pan a few times against the counter to pop any bubbles. Let cool completely. Cover and let sit at room temperature for at least 12 hours, preferably 24, so the caramel has enough time to properly set and harden. (Alternatively, let the caramel cool to room temperature, then cover and refrigerate for 1 hour.)

Make the chocolate ganache:

1. Place 1/2 cup dark chocolate chips, 1/4 cup of the milk chocolate chips, and 2 tablespoons neutral oil in a medium heatproof bowl or double boiler. Fit over a small saucepan of barely simmering water (the bottom of the bowl should not touch the water). Heat, stirring with a heatproof flexible spatula, until the chocolate is melted, 1 to 2 minutes.

2. Remove the bowl from the saucepan. Add the remaining 1/4 cup milk chocolate chips to the bowl and stir until smooth and all the chocolate is melted. Let cool until cool to the touch (about 80℉, just a little warmer than an ideal room temperature of 75℉), about 40 minutes.

3. Pour the ganache over the caramel layer. Spread into an even layer with an offsept spatula, pushing it into all the corners. Tap the pan a few times against the counter to pop any bubbles. Use the offset spatula to make a few swooshes in a decorative pattern if desired. Cover and refrigerate until the chocolate is set.

4. Run a knife around the edges of the pan. Use the parchment paper sling to lift the slab out of the pan onto a cutting board. Slowly and carefully peel off and remove the parchment.

5. Cut into 16 (2 1/4-inch) squares or desired pieces. I recommend gently slicing just into the chocolate, then slicing through the rest of the cookie, so the chocolate doesn’t break. Sprinkle with flaky salt before serving.