Chop the chocolate finely and place it in a heatproof bowl.
Heat the cream in a small saucepan over medium heat until it just begins to simmer; do not boil.
Pour the hot cream over the chopped chocolate and let sit for 60 seconds to soften the chocolate.
Gently stir the chocolate and cream from the center outward until smooth and glossy.
Add the softened butter and stir until fully incorporated for extra sheen and richness.
If using powdered sugar for sweetness or thickness, sift it in now and whisk until smooth.
Stir in vanilla extract and a pinch of salt; taste and adjust sweetness as needed.
For a thinner glaze, add 1–2 teaspoons of warmed cream or milk and whisk to the desired consistency.
To thicken the icing, chill it 5–10 minutes, stirring periodically until spreadable.
If the icing has cooled and stiffened too much, warm gently over a double boiler or microwave in 5–8 second bursts, stirring each time until pourable.
Pour the warm icing over a cooled cake in a slow stream, starting at the center and letting it drip naturally to the edges.
Use an offset spatula to nudge drips or to smooth the top; allow to set at room temperature or refrigerate briefly for a firmer finish.