In a small microwavable bowl, stirring with a silicone spatula every 15 seconds (or in the top of a double boiler set over hot, not simmering, water, stirring often—do not let the bottom of the container touch the water), heat the chocolate until almost completely melted.

Remove the chocolate from the heat source and stir until fully melted. Pour the cream on top of the chocolate and stir until smooth. The mixture should drop thickly from a spoon. If it is too thin, let it cool for a few minutes. Use it at once to pipe or drizzle over the cake. To pipe, pour the glaze into a disposable pastry bag or a quart-size reclosable freezer bag with a very small semicircle cut from the tip or one corner, and close it securely.