Line an 8- by 8-inch baking pan with parchment paper, leaving a 2-inch overhang; set aside.
In a food processor, pulse cookies until finely ground, about 15 pulses. Using a fork, stir sugar and ¼ teaspoon salt into ground cookies. In a small microwave-safe bowl, microwave 8 tablespoons butter on high, stirring occasionally, until melted, about 1 minute. Pour into crumb mixture and stir with a fork until it resembles wet dirt. Transfer crumbs to prepared baking pan. Firmly press crumbs to form a compact, even layer across the bottom of the pan. Freeze, uncovered, until firm, about 15 minutes.
In a large microwave-safe bowl, combine 2 cups of chocolate chips (10 ounces) and 4 tablespoons of butter; microwave on high, stirring every 30 seconds with a whisk, until melted and smooth, about 2 minutes. Stir in sweetened condensed milk. Then, stir in instant espresso, vanilla, and remaining ½ teaspoon salt. Pour mixture into chilled crust. Spread into an even layer with a silicone spatula. Refrigerate, uncovered, until set, about 1 hour.
In a medium microwave-safe bowl, combine the remaining ½ cup of chocolate chips (3 ounces) and the remaining 4 tablespoons of butter. Microwave on high, stirring occasionally with a whisk, until melted, about 1 minute. Stir until completely combined. Pour over chilled chocolate layer. Using an offset spatula, quickly spread into an even layer. Refrigerate, uncovered, until set, about 30 minutes. Lift parchment to remove bars from pan, and transfer to a cutting board. Using a sharp knife, cut into 16 squares. Store in an airtight container in the refrigerator.
Yield 16 squares4