Feel free to substitute finely chopped toasted nuts of any variety if you don't care for coconut. Alternatively, skip the nuts altogether and just make this a two-ingredient treat, upping the amount of crispy brown rice cereal to 1 1/3 cups.
Chop the chocolate into small pieces that are roughly even in size. Put it into a medium, microwave-safe bowl. Microwave the chocolate on high in 30-second bursts, mixing after each round, until the chocolate is melted and smooth. This took 2 1/2 minutes in my microwave. Alternatively, melt the chocolate in a medium pot set over another pot of simmering water, stirring until melted.
Spoon 1 level tablespoon of the chocolate mixture onto the baking sheet. Use your fingers (and perhaps the aid of the tip of a knife) to spread the chocolate into the shape of a heart. Continue with the remainder of the chocolate, leaving an inch or so of space between. If you don't want to go to the trouble of making hearts, just spread the chocolate into one very thin sheet on the baking pan using a rubber spatula.
Leave the chocolate to cool and firm up, about an hour (if you spread it in one big sheet, just snap into 2-bite pieces). You can speed up the process by putting the chocolate in the fridge or freezer.
Store in an air-tight container. Will keep well for 1 week.