Pre-heat oven to 375°F.
Butter a shallow 9"x9" square baking pan (1" - 1 1/2" deep).
Line bottom and two sides with parchment paper, leaving an overhang. Butter parchment.
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.
Spread dough in pan; prick all over with fork.
Bake until golden, 15 - 20 minutes, then cool completely in pan on a rack, about 30 minutes.
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° on a candy thermometer.
Remove from heat and whisk in chocolate until melted.
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 wipe clean between cuts) into 24 bars.
Chill until ready to serve.