Process chocolate in food processor until very fine. Put it in a small heavy saucepan.
Heat cream to boiling point and pour 3/4 of it over the chocolate. Cover for 5 minutes to allow it to melt. Gently stir together, trying not to create air bubbles. Pass through fine strainer and add Cognac. Cool till just tepid.
Check consistency: when tepid, the glaze should mound a bit before disappearing. If it is too thick, or seems curdled, add more of the warm cream little by little. If too thin, add some melted chocolate. When right, use at once or store and reheat.
Pour over cooled cake allowing excess to flow down the sides. If you want a double coat, refrigerate the cake after the first later, for about 20 mins or until firm. Apply a second coat -- but don't refrigerate after second coat or you'll lose that glossiness.