Microwave for 1 1/2 minutes, or until set. Meanwhile, in a small bowl, mix icing ingredients until mostly smooth and uniform. Once cooked, immediately run a knife along the edges to help separate the cake.
Firmly place a plate over the mug, and flip so that the plate is on the bottom. Gently shake mug to release cake onto the plate. Top with icing. MAKES 1 SERVING. HG Tip: For perfectly swirled icing, use a DIY piping bag. Transfer icing to a plastic bag, and squeeze it down toward a bottom corner.
Snip off that corner with scissors, creating a small hole for piping.