Ingredients

• 1 stick unsalted butter, softened
• 1/3 cup packed light brown sugar
• 1/2 teaspoon pure vanilla extract
• 1/2 teaspoon salt
• 1 cup all-purpose flour
• 1 cup heavy cream
• 1 cup dulce de leche
• 4 large egg yolks
• 5 ounces 60%-cacao bittersweet chocolate, finely chopped (preferably in a food processor)

Instructions

1. Preheat oven to 375°F with rack in middle. Butter a shallow 9- to 9 1/2-inch square baking pan (1 to 1 1/2 inches deep). Line bottom and 2 sides with parchment paper, leaving an overhang. Butter parchment.
2. Blend together butter, brown sugar, vanilla, and salt in a bowl with a fork. Sift in flour and blend with fork until a soft dough forms.
3. Spread dough evenly in baking pan using an offset spatula or back of a spoon, then prick all over with fork.
4. Bake until golden, 15 to 20 minutes, then cool completely in pan on a rack, about 30 minutes.
5. Make chocolate dulce de leche: Bring cream and dulce de leche to a simmer in a small heavy saucepan, stirring with a wooden spoon until dulce de leche has dissolved. Whisk together yolks in a bowl, then slowly whisk in hot cream mixture. Return to pan and cook over medium heat, stirring constantly, until pan is visible in tracks of spoon and mixture registers 170°F on an instant-read thermometer. Remove from heat and whisk in chocolate until melted.
6. Bring cream and dulce de leche to a simmer in a small heavy saucepan, stirring with a wooden spoon until dulce de leche has dissolved.
7. Whisk together yolks in a bowl, then slowly whisk in hot cream mixture. Return to pan and cook over medium heat, stirring constantly, until pan is visible in tracks of spoon and mixture registers 170°F on an instant-read thermometer. Remove from heat and whisk in chocolate until melted.
8. Make bars: Pour chocolate mixture over cooled shortbread and chill, uncovered, until cold and set, about 2 hours. Run a small knife around edges to loosen, then transfer to a cutting board using parchment. Cut with a hot clean knife (dip in hot water and wipe clean between cuts) into 24 bars. Chill until ready to serve.
9. Pour chocolate mixture over cooled shortbread and chill, uncovered, until cold and set, about 2 hours.
10. Run a small knife around edges to loosen, then transfer to a cutting board using parchment. Cut with a hot clean knife (dip in hot water and wipe clean between cuts) into 24 bars. Chill until ready to serve.