Pour maple syrup and 1 1/2 cups water in a medium sauce pan. Warm over medium heat until just simmering. Whisk in cocoa powder (it will take a minute or two to get all of the cocoa completely incorporated). Remove from heat and whisk in vanilla. Pour through a fine mesh sieve into a glass jar. Store in the refrigerator for up to 3 weeks. The ingredients will settle a bit while in the refrigerator, so make sure to stir or shake the container before using.