Line a 13- x 9-inch pan with aluminum foil; grease foil. Set aside.
Beat 1 cup brown sugar, melted butter, eggs, 1 teaspoon vanilla, and if desired, bourbon at medium speed with an electric mixer until smooth. Combine flour, cocoa, and 1/2 teaspoon salt. Gradually add to brown sugar mixture, beating until blended. Spread into prepared pan.
Bake at 375° for 15 minutes. Cool on a wire rack.
Bring 3/4 cup butter, syrup, remaining 3 cups brown sugar, and remaining 1/2 teaspoon salt to a boil, stirring constantly; boil, stirring constantly, until a candy thermometer registers 250° (hard ball stage). Remove from heat; stir in whipping cream and 2 tablespoons vanilla until blended. Stir in pecans. Pour over prepared crust.
Bake at 375° for 25 minutes. Cool on a wire rack; cover and chill 8 hours. Cut into 12 squares; cut each square into 2 triangles. Store in refrigerator.