In a small heavy saucepan,melt the butter over mod. low heat;add the cocoa and cook the mixture, whisking for 30 seconds. Whisk in the sugar and cook the mixture for 15 seconds. Whisk in a pinch of salt, the vanilla and the yogurt and cook sauce for 12 to15 seconds or until hot, but do not let boil. Serve the sauce over fruit or ice cream. Makes 1/2 cup.