Place all of the ingredients in a large bowl and whisk together well to combine. Whisk the mix again before measuring. It’s important to use table salt in the cake mix; other types will eventually settle out of the mix.
For turning it into Vanilla Cake:
Preheat the oven to 350° F. Butter the bottom and side of the pan(s). Dust with flour to coat, then invert and tap out any excess. (If making cupcakes, use liners instead of greasing and coating the tins.)
Place the cake mix in a large bowl. Stir in the yogurt, butter, water, vanilla, and eggs until moistened and no lumps remain (be careful not to overmix). For Confetti Cake, stir 1/2 cup rainbow sprinkles into the batter just before pouring it into the pans. Divide the batter between the prepared pans.
Bake until the layers are domed and golden brown, and a few moist crumbs cling to a skewer inserted in the center of the cake, 35 to 40 minutes (40 to 50 minutes for a Bundt, 25 to 30 minutes for a 13 by 9-inch cake, and 20 to 25 minutes for cupcakes).
At this point, coat the layers with syrup (https://food52.com/recipes/60584-vanilla-syrup-mixed-berry-syrup-sweet-cream-syrup-variations). After removing the hot cake layers from the oven, pierce them, still in their pans, at one-inch intervals with a skewer or paring knife. This creates channels for the syrup to seep into the cakes. Then, pour or generously brush the syrup over the surface of the hot layers, dividing it between them as evenly as you can. Transfer the soaked layers (still in their pans) to a wire rack to cool completely.
When they are cooled and no longer wet to the touch, one to two hours, carefully turn them out of their pans and assemble and frost.
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.