Hosts Julia Collin Davison and Bridget Lancaster uncover the secrets to the most decadent bar cookie recipe, Millionaire’s Shortbread on Season 18 of America’s Test Kitchen.

Makes 40

INGREDIENTS

FOR THE CRUST:

  • 2½ cups (12½ ounces) all-purpose flour
  • ½ cup (3½ ounces) granulated sugar
  • ¾ teaspoon salt
  • 16 tablespoons unsalted butter, melted

FOR THE FILLING:

  • 1 (14-ounce) can sweetened condensed milk
  • 1 cup packed (7 ounces) brown sugar
  • ½ cup heavy cream
  • ½ cup corn syrup

  • 8 tablespoons unsalted butter
  • ½ teaspoon salt

FOR THE CHOCOLATE LAYER:

  • 8 ounces bittersweet chocolate (6 ounces chopped fine, 2 ounces grated)

TO PREPARE

CRUST
STEP ONE. Adjust oven rack to lower-middle position and heat oven to 350 degrees. Make foil sling for 13 by 9-inch baking pan by folding 2 long sheets of aluminum foil; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan.

STEP TWO. Combine flour, sugar, and salt in medium bowl. Add melted butter and stir with rubber spatula until flour is evenly moistened. Crumble dough evenly over bottom of prepared pan. Using your fingertips and palm of your hand, press and smooth dough into even thickness. Using fork, pierce dough at 1-inch intervals.

STEP THREE. Bake until light golden brown and firm to touch, 25 to 30 minutes. Transfer pan to wire rack.

STEP FOUR. Using sturdy metal spatula, press on entire surface of warm crust to compress (this will make finished bars easier to cut). Let crust cool until it is just warm, at least 20 minutes.

FILLING
STEP FIVE. Stir all ingredients together in large, heavy-bottomed saucepan. Cook over medium heat, stirring frequently, until mixture registers between 236 and 239 degrees (temperature will fluctuate), 16 to 20 minutes. Pour over crust and spread to even thickness (mixture will be very hot). Let cool completely, about 1½ hours.

CHOCOLATE LAYER
STEP SIX. Microwave chopped chocolate in bowl at 50 percent power, stirring every 15 seconds, until melted but not much warmer than body temperature (check by holding in palm of your hand), 1 to 2 minutes. Add grated chocolate and stir until smooth, returning to microwave for no more than 5 seconds at a time to finish melting if necessary. Spread chocolate evenly over surface of filling. Refrigerate shortbread until chocolate is just set, about 10 minutes.

STEP SEVEN. Using foil overhang, lift shortbread out of pan and transfer to cutting board; discard foil. Using serrated knife and gentle sawing motion, cut shortbread in half crosswise to create two 6½ by 9-inch rectangles. Cut each rectangle in half to make four 3½ by 9-inch strips. Cut each strip crosswise into 10 equal pieces. (Shortbread can be stored at room temperature, between layers of parchment, for up to 1 week.)