Whisk together cocoa powder and water in a medium saucepan over medium heat. When mixture is smooth, whisk in maple syrup. Increase heat to medium-high and whisk constantly until mixture comes to a low boil.
Reduce heat to medium and continue whisking until syrup thickens slightly, about 30 seconds. Remove from heat and stir in chocolate until melted. Cool to room temperature.
Put hazelnuts in the bowl of a food processor. Blend for 2 to 3 minutes, slowly adding hazelnut oil through the feed tube as you go, until you have a thin nut butter.
Transfer hazelnut butter to a measuring cup with a pouring spout. Pour cooled chocolate syrup into food processor bowl (no need to wash it out first). Blend syrup and, with the machine running, pour hazelnut butter through the feed tube in a steady stream until you have a creamy, nutty spread. It will be thin but will firm up in the refrigerator.