I think the sugar has to "melt"-- the temperature of whatever the sugar is stirred into needs to be high enough to dissolve the sugar.

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Melt some coconut oil with your sugar first, and thin your chocolate with coconut oil.
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I tried it today with powdered sugar, much better. Not grainy.

Melt the sugar FIRST in a sauce pan. Then you can add the other ingredients, i.e., evaporated milk and chocolate, butter, flavoring, etc. NEVER add water to chocolate. Good luck.