What caused smooth homemade nutella to become grainy in refrigerator? I blended toasted hazelnuts in a vitamix until they were a butter, added melted chocolate, then added coconut oil to increase smoothness. Here are the amounts:

450 grams hazelnuts (unsalted, raw or roasted)
160 grams chocolate
50 grams coconut oil

Once it was blended, the texture was very smooth and silky. 

After a day or two in the refrigerator, the nutella had become grainy.

When you taste it the graininess subsides as it melts in your mouth. This leads me to believe that the graininess is caused by hardened bits of oil, probably the coconut oil.

How can I further emulsify the ingredients to avoid this?
You're right, it's the coconut oil. Notice how shiny the batch was fresh? That's the oil separating. It would need constant stirring while cooling to keep droplets from forming. Steady cooling  - I got that graininess in ice cream once by freezing too quickly from room temp.

Got an ice cream churner around to try with just fridge temp?