Preheat oven to 350°F (180°C). Butter three 8-inch round cake pans and line with parchment. Whisk melted butter and sugar until smooth. Beat in eggs one at a time, then vanilla. Sift in cocoa, flour, and salt; fold gently just until combined. Divide batter among pans, top with chocolate chips if using, and bake 18–20 minutes until just set. Cool in pans 10 minutes, then turn out to cool completely.

Place chopped chocolate in a heatproof bowl. Heat cream until steaming, pour over chocolate, let sit 2 minutes, then stir until smooth. Chill at least 2 hours until thick but spreadable.

Whip chilled ganache with an electric mixer on medium speed until fluffy and light, about 1 minute. Do not overwhip.

Place one brownie layer on a plate, spread with a third of whipped ganache. Repeat with remaining layers. Frost top and sides with remaining ganache. Decorate if desired, then chill 30 minutes before slicing.