In a medium saucepan, combine 2 cups water and the sugar and bring to a boil over high heat, stirring occasionally. Reduce the heat to low and gradually add the cocoa powder, whisking until smooth. Cook the mixture at a gentle simmer for 30 minutes, until syrupy.
Put the chocolate in a large bowl and add half the cocoa syrup, whisking until the chocolate is melted and the mixture is smooth. Add the remaining syrup and 2 cups water and whisk well. Strain the mixture through a fine sieve and let cool.
Chill the sorbet mixture, covered, until very cold, at least 4 hours, and up to 2 days. Freeze in an ice-cream maker according to the manufacturer's directions. Serve with Candied Kumquats.
In a medium saucepan, combine the sugar and 1 1/2 cups water. Bring to a boil, stirring to dissolve the sugar. Add the kumquats, reduce the heat to low, and simmer for about 6 minutes, or until the kumquats appear translucent. Drain well before serving. The kumquats will keep, refrigerated, for several weeks.