To make sorbet without an ice cream maker: Make the chocolate mixture, but omit the alcohol. Place the mixture in a shallow pan; an 8" x 8" square pan or 9" round cake pan are both good choices. Place the pan in the freezer. There's no need to cover it.
After 2 hours, use a fork or spoon to stir it around, bringing the frozen edges into the center. Return to the freezer.
Continue to stir every hour or so, until the sorbet is nearly as firm as you like. This may be as little as 4 hours total, start to finish; or it may take longer, depending on the temperature of your freezer.
Once the sorbet is entirely icy (like a slush drink), add the alcohol, if you're using it. Rather than add the alcohol while the sorbet is still in its shallow pan, transfer the sorbet to a lidded storage bowl. Stir in the alcohol, then cover the bowl and place it in the freezer. Let the sorbet "ripen" (freeze), undisturbed, for several more hours, until it's as hard as you like.