1 1/2 cups granulated sugar
1/4 cup light corn syrup
1/4 cup water
1/2 cup heavy cream
3 tablespoons unsalted butter, softened
1/4 teaspoon salt
Baking spray with flour
1 1/2 cups unsalted butter
4 large eggs
2 1/2 cups packed light brown sugar
1 teaspoon vanilla extract
3 1/2 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon salt
3/4 cup chopped walnuts, divided
1 (4-oz.) semisweet chocolate bar, chopped, divided
Prepare the Caramel: Bring granulated sugar, corn syrup, and water to a boil in a medium saucepan over medium-high. Boil, undisturbed, until mixture turns amber in color, about 10 minutes. Remove from heat; carefully stir in cream, butter, and salt until smooth. Cool until slightly thickened, about 30 minutes.
Prepare the Blondies: Preheat oven to 350°F. Coat a 13- x 9-inch baking pan with baking spray with flour. Melt butter in a medium saucepan over medium, swirling occasionally, until butter browns and has a nutty aroma, about 10 minutes. Transfer to a large bowl; cool slightly, about 30 minutes. Whisk eggs, brown sugar, and vanilla into brown butter until shiny, about 30 seconds. Stir together flour, baking powder, and salt in a separate bowl until combined. Gradually add flour mixture, 1⁄2 cup of the walnuts, and 2 ounces of the chocolate to brown butter-egg mixture, stirring until combined. Spoon half of the batter into prepared pan. Bake 10 minutes.
Remove pan from oven; drizzle with 1 cup cooled Caramel. Spoon remaining batter over top. Sprinkle with remaining 1⁄4 cup walnuts and 2 ounces chocolate. Return to oven; bake at 350°F until mixture is golden and set, about 25 minutes. Cool completely in pan, about 1 hour. Slice into 32 bars. Drizzle with remaining Caramel just before serving.