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Coffee
If you are making a chocolate cake, add a shot or two of espresso or strong black coffee to make your cake taste even more chocolatey.
Buttermilk
Instead of water, use buttermilk! It will add richness and a make the texture a little denser and wonderfully moist.
Sour Cream
Like with buttermilk, using sour cream instead of the water listed will result in a delightfully dense and moist cake that tastes much more homemade.
Butter
Replace any canola oil the mix calls for and use melted butter instead.
Salt
All desserts benefit greatly from a pinch of salt! Always add a generous pinch to give your cake a great depth of flavor and to cut the sweetness, which can make your cake taste charming and delicate rather than treacly and overpowering.
Extracts and Flavorings
Vanilla extract, lemon zest, orange zest — whatever you fancy! Adding your own flavoring to a boxed cake mix will allow you to turn a vanilla cake into almost anything. Rather than buying a store-bought lemon cake mix, use a white or yellow box cake, and add the juice and zest of a fresh lemon for a citrusy flavor that is far superior to anything found in a box!