You can add powdered sugar to ganache to thicken it, although you probably don't need to. When warm, ganache is thin and pourable, but as it cools, it becomes a thick filling. Powdered sugar changes the color and flavor of the ganache; to avoid that and create a dense frosting with a rich chocolate flavor, use something other than powdered sugar to thicken it.
To thicken ganache with powdered sugar, allow the ganache to cool for at least two hours. Sift the powdered sugar first to remove any lumps and add it to the ganache. Beat with a hand mixer or stand mixer until smooth. The powdered sugar lightens the color of the ganache and adds sweetness.
Make a Change
To thicken ganache without using powdered sugar, use less heavy cream. Recipes typically call for equal parts of cream and chopped chocolate. By using a bit less cream, the ganache is naturally thicker. However, if you use dark chocolate with a cacao percentage over 52 percent, you must add extra cream or the ganache may break and look curdled. Heat the whipping cream just to boiling and pour it over finely chopped chocolate. Whisk it gently until the chocolate melts and the ganache is creamy and smooth. If you whisk it too much, it may separate and become grainy.
Immediately after you make ganache, it is a pourable glaze. Ganache naturally becomes thicker as it cools because the chocolate solidifies. Allow it to sit at room temperature for 8 to 10 hours, and it becomes a thick frosting. If you cover and refrigerate ganache, it becomes solid enough to use in truffles. You can even gently reheat ganache to thin it again, if necessary. Although allowing ganache to cool to thicken takes some advance planning, you'll probably be happier with the results. The ganache will have a creamy texture and deep chocolate color without the cloying sweetness powdered sugar can add.
Whip It Up
For the ultimate ganache frosting or filling without powdered sugar, cover and refrigerate it overnight until it's very thick. Beat the ganache with a hand mixer or a stand mixer, using the paddle blade. The ganache retains its rich chocolate taste, but becomes thick and airy. You can spread it smoothly over a dessert or fill a piping bag and decorate with it.