Seizing—the nearly instantaneous transformation of melted chocolate from a fluid state to a stiff, grainy one—is usually the result of a tiny amount of moisture being introduced.
In recipes that contain no liquid, don’t let moisture get into the melted chocolate. Even a tiny amount will form a syrup with the sugar in the chocolate to which the cocoa particles will cling, creating grainy clumps. In recipes containing liquids such as melted butter, liqueur, or water, always melt the chocolate along with these ingredients.
Surprisingly, adding liquid will return seized chocolate to a fluid state, as the liquid dissolves seized sugar and cocoa particles. Add boiling water to the chocolate, 1 teaspoon at a time, stirring vigorously after each addition, until smooth. But don’t use the diluted chocolate for baking; use it for chocolate sauce or hot chocolate.