Finely chop chocolate into small even pieces for smooth melting.
Make ganache by mixing chopped chocolate, butter, salt, coffee, and cream in a bowl over simmering water until glossy.
Cool ganache fully at room temperature.
Whip cream with sugar and vanilla in a chilled bowl until soft peaks form.
Add one-third of ganache into whipped cream and fold or whip gently.
Add remaining ganache and mix until smooth and even.
Spoon mousse into cups or bowls, cover, and refrigerate for 4 hours.
Garnish with berries, mint, or chocolate shavings before serving.
