Ingredients

• 1 cup (2 sticks) unsalted butter, room temperature, plus more for dish
• 1/2 cup (packed) light brown sugar
• 1/4 cup sugar
• 1/2 teaspoons fine sea salt
• 1 1/2 teaspoons vanilla extract
• 1 1/2 cups all-purpose flour
• 6 ounces semisweet or bittersweet chocolate or high-quality milk chocolate, finely chopped
• 1-1 1/2 cups assorted toppings, such as cocoa nibs, crushed candy, toasted chopped almonds and pistachios, lightly toasted coconut, and popcorn
• Flaky sea salt (such as Maldon)

Instructions

1. Arrange a rack in middle of oven; preheat to 375°F. Line the bottom and sides of a 13x9x2" metal or glass baking dish with foil, allowing 2" overhang on either side; butter foil in dish.
2. Using an electric mixer at medium speed, beat 1 cup butter in a large bowl until smooth, about 3 minutes. Add both sugars and salt; continue to beat until mixture is light and creamy, about 3 minutes longer. Beat in vanilla, then slowly mix in flour, beating until entirely incorporated (dough will be wet and sticky).
3. Scrape dough into prepared dish. Using your fingertips, spread into a thin, even layer.
4. Bake cookie base until it is golden brown and has begun to puff and crack, about 22 minutes. DO AHEAD: Base can be made 2 days ahead. Let cool completely and store airtight at room temperature.
5. For topping: Preheat oven to 375°F. Scatter chopped chocolate evenly over cookie base and bake just until chocolate is soft and has begun to melt, about 2 minutes. Immediately spread chocolate in an even layer over base. Scatter toppings over warm chocolate; lightly but firmly press into chocolate. Sprinkle with salt. Let cool in dish on a wire rack for 15 minutes. Using foil overhang, lift cookie from dish. Place on rack; let cool until chocolate is set, about 2 hours. Carefully remove foil from cookie and slide onto a cutting board. Cut into bars. DO AHEAD: Crunch Bars can be made 5 days ahead. Store airtight at room temperature.
6. Preheat oven to 375°F. Scatter chopped chocolate evenly over cookie base and bake just until chocolate is soft and has begun to melt, about 2 minutes. Immediately spread chocolate in an even layer over base.
7. Scatter toppings over warm chocolate; lightly but firmly press into chocolate. Sprinkle with salt. Let cool in dish on a wire rack for 15 minutes. Using foil overhang, lift cookie from dish. Place on rack; let cool until chocolate is set, about 2 hours.
8. Carefully remove foil from cookie and slide onto a cutting board. Cut into bars. DO AHEAD: Crunch Bars can be made 5 days ahead. Store airtight at room temperature.