Adding water or a similar liquid (cream, even butter) when melting chocolate will indeed prevent seizing. Twenty percent may work, but if you are melting fine quality dark chocolate which contains a higher amount of cocoa solids (usually 55-65%) you will need a little more liquid. I’d suggest starting with 25%, by weight or roughly 1 tablespoon of liquid for 2 ounces of chocolate. Cocoa powder, being all cocoa solids, would require even more liquid.

Gary is right in his assumption that temperature matters. Whenever possible I dissolve cocoa powder in boiling water. To bring out the deepest chocolate flavor, try cooking the cocoa in the water, at just the lowest simmer, for a minute or so before adding any other ingredients.

It may sound odd, but it does seem to matter whether you add chocolate to the hot liquid or hot liquid to the chocolate. Most ganache recipes, for example, instruct you to boil the cream and pour it over chopped chocolate. I sometimes find, depending on the chocolate and how finely it has been chopped, that this leaves little specks of unmelted (seized) chocolate. Stirring the chocolate into the hot cream, however, is less likely to create specks. I’m not sure why this should matter, but I suspect it has to do with how rapidly the cream cools off which would obviously affect its ability to melt the chocolate. Once the specks form they are not easily dissolved, even if you reboil the mixture. And, as you have discovered, straining them out reduces the chocolate flavor.

Hope this helps. Cindi