Put the cream in a microwave-safe small bowl and heat at 50% power until good and hot, 1 1/2-2 minutes. then stir in the chocolate until melted and smooth. Let cool slightly, then use your faithful disher to put a little dab on each brownie. They will each have a small sunken well, so aim to just fill that up. Of course, if you baked the brownies in a pan then you'll just spread it on like any other frosting. You don't want the brownies to be warm, but you do want the ganache to be slightly so, as it hardens as it cools.