Place the chocolate in a small heat-proof bowl. Heat the glaze in a small saucepan over medium heat until it just begins to steam. Pour over the chocolate and let sit for 3 minutes. Whisk until mixture is smooth.
Let cool for 5 to 10 minutes (until it’s thick but still pourable) then pour over the frosted cake.
Alternatively, you can pour the glaze into a plastic squeeze bottle and squeeze a circle of glaze around the edge of the cake. Squeeze a little more glaze out in some places to create drips. Pour the remaining glaze over the cake and smooth the top.
Refrigerate to set the glaze then slice and serve.