For the crust:
1-1/2 cups all-purpose flour
3/4 cup confectioners sugar
1/4 teaspoon fine sea salt
6 ounces (12 tablespoons) chilled unsalted butter, cubed
1/2 teaspoon vanilla bean paste or pure vanilla extract
For the filling:
1 pound good-quality semisweet chocolate, chopped
3/4 cup whole or 2% milk
4 ounces (8 tablespoons) unsalted butter, cubed
4 large eggs
2/3 cup granulated sugar
3 tablespoons Kahlúa (or 2 teaspoons vanilla)
- For the crust, preheat the oven to 350°F. Grease the bottom and sides of a 9 x 13-inch baking pan. Line the pan with parchment paper, leaving a 1-inch overhang on each side to allow for easy removal.
- Combine flour, confectioners sugar, and salt in the bowl a food processor and pulse to combine. Add the butter and vanilla and continue pulsing until the mixture is crumbly and coarse, about 45 seconds.
- Transfer the mixture to the prepared pan and press into a smooth layer. Bake until the crust is pale golden and the edges just begin to caramelize, 20 to 22 minutes. The shortbread crust will continue cooking in the pan so be careful not to overcook, which will result in a hard crust. Cool completely.
- For the filling, lower the oven temperature to 325°F.
- Combine the chocolate, milk, and butter in a heavy saucepan and cook on low heat, whisking occasionally, until the chocolate is just barely melted. Remove from heat and continue whisking until smooth.
- In the bowl of a stand mixer fitted with the paddle attachment (or with a handheld electric mixer) beat the eggs, sugar, and Kahlúa on medium speed until foamy and light, about 2 minutes. Add the melted chocolate mixture and continue mixing until combined, about 30 seconds.
- Pour the chocolate mixture over the cooled shortbread crust and use an offset spatula to spread evenly. Bake for 30 minutes, until the sides are slightly puffed and a toothpick inserted near the center comes out wet and gooey. The chocolate may still seem a bit underdone, but it will continue cooking out of the oven. (The center may sink a little bit; the original recipe says to use your fingertips to gently press the edges down to the level of the center, but I just leave it be, and later cut the edges off for snacking.)
- Allow the bars to cool completely, then cover with plastic wrap and refrigerate until very cold, at least 12 hours (or up to 2 days). Allow to sit at room temperature for 5 to 10 minutes to take the chill off slightly.
- Using the overhanging paper as handles, remove the bars from the baking dish, using a large spatula if necessary to help lift. Place the bars on a cutting board and carefully peel off and discard paper. Using a sharp knife dipped in hot water, first cut away the edges of the bars to form an even rectangle. (I like to snack on these bits and pieces!)
- Cut the bars lengthwise into 1 1/4-inch strips, then into 1 1/4-inch strips crosswise to form squares, wiping the knife clean with a damp towel after every cut. Cut each square diagonally to make triangles. (If the chocolate seems to be getting a little soft, refrigerate the strips for a few minutes before continuing on.)
- Refrigerate the bars, covered, until ready to enjoy; pull 5 minutes before serving.