Combine ingredients in a large heatproof bowl set over a saucepan of simmering water. Stir with a silicone spatula until glossy and completely melted, 3-5 minutes.
Serve immediately, spooned over ice cream or cake, or transfer to a jar. Sauce will keep, refrigerated, for up to 2 weeks. (The mixture will solidify when chilled; reheat before serving.)