Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Melt butter and chopped chocolate in a bowl over simmering water, stirring until smooth.
Set aside 3 minutes to cool slightly.
Whisk in granulated sugar and brown sugar until glossy and combined.
Beat in eggs one at a time until fully incorporated.
Stir in vanilla, whiskey, espresso powder, and salt.
Sift flour and cocoa over the bowl. Fold gently just until no dry streaks remain.
Fold in chocolate chips and walnuts, if using.
Spread batter into the pan and smooth the top.
Bake 28–32 minutes, until the center is set with moist crumbs on a tester.
Cool in the pan at least 30 minutes.
Whisk powdered sugar, cocoa, whiskey, and cream until smooth and pourable. Add a splash of cream if needed.
Pour or spread glaze over brownies. Let set 20 minutes, then slice and serve.