Combine the butter, malted milk powder, salt, and 2 cups of the sugar in a large bowl and beat with an electric mixer on low speed until incorporated, about 1 minute.
Add the remaining sugar and beat on medium speed until the frosting is pale and no longer grainy, about 2 minutes. Add the vanilla and beat until the frosting is very light and fluffy, about 2 minutes.
Malted Vanilla Frosting will keep, in an airtight container in the refrigerator, for 1 week. Before using, bring it back to room temperature and stir it vigorously or beat it again for best results.
Before her diagnosis, Caroline wrote a book on cakes called Cake Magic!. She started developing a birthday cake using her gluten-free mix found in that book. Check out other recipes she’s developing for her new life—and the stories behind them—on her blog, The Wright Recipes. Her next book, Catalan Food, written with chef Daniel Olivella, comes out in early September from Clarkson Potter.