Chocolate Caramel Dessert Bars

For the Caramel Layer: In a medium heavy-bottomed saucepan, combine granulated sugar, brown sugar, cream, butter, and corn syrup. Cook over medium-low heat, stirring occasionally with a wooden spatula, until the butter melts and mixture is smooth. Continue to cook, stirring occasionally at first, then more frequently as it begins to darken and thicken, until it turns a deep golden brown and reaches 248°F (120°C), 25 to 30 minutes. As it cooks, you should see the mixture gradually thicken, darken in color, and form steady bubbles; begin checking the temperature more frequently after the first 20 minutes to avoid overcooking. When caramel has reached 248°F (120°C), immediately remove from heat. Add salt and evaporated milk, stir to combine. Transfer caramel to a heatproof bowl. Let cool at room temperature until slightly thickened but still pourable, about 30 to 45 minutes, before using.

For the Dough: Meanwhile, in a large bowl, whisk together flour, sugar, baking powder, and salt; set aside.

In a small saucepan, melt 8 tablespoons (113 g) butter over medium heat. Transfer to a small heatproof bowl and add the remaining 6 tablespoons (85 g) butter; let sit until melted, about 5 minutes.

In a medium bowl, combine Dutch-process cocoa powder and 4 ounces (113 g) finely chopped chocolate. Add 1/4 cup boiling water and stir with a spoon until a thick chocolate paste forms (a few small unmelted pieces are OK). Set aside.

Adjust oven rack to middle position and preheat oven to 350°F (175°C). Grease a 13- by 9-inch metal baking pan with remaining 1/2 tablespoon softened butter; set aside.

To the bowl with the dry ingredients, add the melted butter, chocolate paste, evaporated milk, and vanilla extract. Using a silicone spatula, fold until a cohesive chocolate dough forms and no dry pockets remain.

Press about two-thirds of the dough (24 ounces; 680 g) evenly into the prepared pan, set aside, and reserve the remaining dough (about 12 ounces; 340 g) for the topping. Bake until just set, 8 to 12 minutes.

Let pan cool for 5 minutes, then sprinkle the remaining 4 ounces of roughly chopped chocolate evenly over the partially baked base. Pour the caramel over the top in an even layer. Dollop the remaining dough over the caramel; it does not need to cover the surface fully.

Return pan to oven and bake until topping is set and the edges are lightly darkened, 16 to 20 minutes.

Let cool at room temperature for 15 minutes, then refrigerate until the caramel is fully set, at least 3 hours or up to 24 hours. Sprinkle with flaky sea salt, if using, then cut into 32 squares and serve.