Hi! Grittiness can happen if the white chocolate gets too hot or hasn’t melted completely. Try gently warming the syrup over low heat, whisking in a little warm milk, about a tablespoon at a time, until it smooths out. Avoid letting it boil. If the chocolate scorched, it unfortunately may not become completely smooth again.

What brand of white chocolate did you use, and was it gritty right away or only after cooling? That will help me narrow it down!