There are two ways to do this, and both work equally well. I tend to heat up the cream rather than using a double-boiler out of sheer laziness.
<b>Option 1:</b> Gently heat the cream in a microwave-safe bowl or a small saucepan on the stove. You want it hot (steaming lightly) but NOT boiling. Add the chocolate chips and instant coffee and whisk until smooth and creamy. Be patient, it will look weird for a bit but just keep whisking until gets all glossy.
<b>Option 2:</b> Use a double-boiler (a bowl on top of a saucepan with simmering water) to slowly melt the chocolate, cream, and instant coffee together. You want the water simmering, not boiling. Whisk consistently until the chocolate has melted and the entire mixture is smooth. Don't rush it, it'll happen.
Pour on the cake immediately, then let it set for a bit before cutting.