Stewart was tempering chocolate, which is the process of melting chocolate in a way that allows it to harden again when it cools. Her demo put me off working with chocolate for a decade. If done wrong, the melted chocolate won't firm up when it cools. Tempering is the secret to everything from getting a thin coat of crisp-hard chocolate on truffles to making chocolate bark. It's also relatively easy to temper chocolate with a bowl, a spatula, and a microwave.
Many sources suggest that melting chocolate in a microwave requires the cook to put the chocolate in for 15 seconds on high, stir it, and repeat until melted - which often require as many as ten repetitions. If you let it go for 30 seconds the chocolate burns. The secret to a much less onerous process is to melt it on the defrost setting - which is power level 3. Then you can let it melt for two or three minutes and it won't burn. Stir it once and put it back in on defrost for another minute or two.
Melting the chocolate is the first half of the tempering process. The second half is seeding the chocolate. The easiest way to do this is to put a little unmelted chocolate into the bowl with the just-melted chocolate, and stir until it melts too. Generally that's allows you to achieve a temper. To hedge my bets I like to stir for an extra minute or two, and as I do that I hold the spatula a foot above the bowl and allow the melted chocolate to flow like a ribbon into the pool of melted chocolate. This extra bit of assisted cooling ensures a temper.