I googled recipes and the first few recipes I found made larger quantities than I thought I would want to commit to. I did find this one that looked a little more reasonable.
½ cup cocoa powder
1 cup water
2 cups sugar
⅛ teaspoon salt
¼ teaspoon vanilla
I didn't have any white sugar (weird, I know!), so I substituted brown sugar. Seemed to work out okay.
Pay special attention to the tip to be careful and not let it boil over! It was quite a big mess to clean up!