Preheat oven to 350°F Grease a 9x13" pan and line bottom and long sides with parchment paper overlapping sides.
Warm unsweetened chocolate and butter in saucepan over medium heat until chocolate is just melted. Whisk until smooth and remove from heat.
Stir in sugar, then eggs, and 2 tbsp of the whiskey. Gradually stir in the flour and salt.
Pour into prepared pan and bake until almost set (toothpick inserted into center should have moist crumbs clinging to it) about 25 to 30 minutes. DO NOT OVERBAKE.
Let cool completely. Loosen edges at side with knife and using parchment paper carefully lift brownies from pan.
Bring cream to a simmer and add bittersweet chocolate. Turn off heat and and let stand until chocolate is softened. Whisk until smooth; then whisk in the remaining 2 tbsp whiskey. Drizzle over brownies and spread to smooth. Let cool completely before cutting.