Step 1

Plan the Dulce de Leche: If you can find dulce de leche at the store, you may use that instead, but usually you have to cook your own from cans of sweetened condensed milk. The safest way: Place the sweetened condensed milk in a double boiler setup, by setting a glass heatproof bowl on top of a saucepan with two inches of simmering water. Make sure the water doesn't touch the bottom of the bowl. Cook for 3 hours, until thick and butterscotch colored, stirring every hour. You may need to refill the water once or twice. Let cool.The impatient, riskier way: You can cook the sweetened condensed milk in a nonstick saucepan over medium heat for about 15-20 minutes until thick and caramelized, but you MUST stir constantly with a spatula, or it will burn. Let cool.

Step 2

Preheat the oven to 350 degrees F.

Step 3

For the shortbread: it is best to use a stand mixer if you have one, but this may all be done with a hand mixer as well. In a stand mixer fitted with the paddle attachment, spread the butter around the bowl by mixing for about 10 seconds on medium. With the mixer still on, pour in the sugar, and beat until the sugar and butter are blended. Briefly beat in the egg yolk until incorporated, then mix in the flour, scraping down the sides of the bowl as needed, until a crumbly dough forms.

Step 4

Line a 9×13 inch metal baking pan with parchment paper, then press the dough out evenly using your fingertips. Prick the dough all over with a fork, and bake for 20-22 minutes, until lightly golden brown. Let the shortbread cool completely on a wire rack, about 15 minutes.

Step 5

Spread the dulce de leche evenly on top of the shortbread layer, then refrigerate for 30 minutes.

Step 6

For the chocolate layer: In a heatproof bowl, combine the dark chocolate chips and butter, and microwave in 30 second intervals until it's almost smooth. Err on the side of microwaving less, so you don't risk overheating the chocolate. I pull it out when the chocolate chips are almost melted, and usually if I keep stirring for a minute, the residual heat will melt it to a smooth consistency. Pour the mixture over the chilled dulce de leche layer and spread it evenly with a spatula. Refrigerate for an hour until the chocolate glaze hardens.

Step 7

Once the bars have chilled, cut into squares and serve. Enjoy!

Step 8

Keep leftovers in the refrigerator for up to one week.