In top of double boiler, over hot (not boiling) water, heat coconut milk. Add chocolate. Stir until chocolate is melted and smooth. Remove from heat.
In mixer bowl with whisk attachment, beat reserved bean liquid and cream of tartar on medium-high speed until foamy, 5 minutes. Gradually add sugar, beating until stiff peaks form.
Add one third of the whipped mixture to the melted chocolate. Whisk until well blended.
Gently fold in remaining whipped mixture, one half at a time.
Cover. Refrigerate overnight.
Notes
Garbanzo bean liquid should be at room temperature before whipping.