Preheat oven to box temperature. Line 12-cup muffin tin or lightly spray if unlined.
Prepare brownie batter per box; stir in optional vanilla and pinch of salt.
Half-fill cups with batter; press 1 tsp chocolate chips or truffle filling into centers. Add batter to 3/4 full.
Bake 16–20 minutes until edges set but centers remain soft; don’t fully harden.
Meanwhile, heat cream until steaming; pour over chopped chocolate and stir into smooth ganache.
When cool enough to handle, poke small holes in cupcakes and spoon warm ganache into centers, or pour over tops.