While I love modifying recipes (and in fact nearly never follow a recipe exactly), it's more complicated to modify baked goods. A good cake depends on a lot of specific chemical reactions, and if you mess with the recipe too much you run the risk of ruining some of that science.
That said, modifying recipes for baked goods is possible. (Even if you, like me, did not particularly enjoy high school chemistry class.) Having a good understanding of how to modify baking recipes is particularly helpful when you decide to bake something on a whim and are missing some ingredients.
To make this delicious triple citrus cake with orange, lemon, and lime, I started with Ina Garten's recipe for Lemon Cake. I knew I didn't have enough lemons, so I decided to use all three types of citrus instead.
Here are the substitutions I made to the original recipe:
The recipe calls for lemon zest and lemon juice. Instead, I zested and juiced lemons, limes, and oranges. This switch works because all citrus zest and juice is similar enough chemically. I made this switch for the cake batter and the syrup. Caveat: if you are using a significantly larger proportion of orange juice, reduce the sugar in the syrup to taste - orange juice is a lot sweeter than lemon and lime juice!
The recipe calls for buttermilk, which I didn't have on hand. In this cake, the acid in the buttermilk reacts with the base in the baking soda, which creates carbon dioxide that helps the cake rise and stay fluffy. Because of this, any substitute for buttermilk needs to also have acid, so you can't switch it for regular milk. To make fake buttermilk, I put 1 tablespoon of white vinegar or lemon juice (both acidic) into a 1-cup measuring cup and then fill the rest with milk - this makes one cup of imitation buttermilk.
The recipe makes a citrus glaze to drizzle over the top of the cake. Instead, I wanted to use up some extra cream cheese so I made a cream cheese frosting. I didn't want it to be too sweet, so I whipped the cream cheese and added powdered sugar 1/2 cup at a time until it tasted good. (See, some aspects of baking can be just as free-form as cooking!)
When you're making cake, especially pound cake like this, your ingredients must all be at room temperature. Seriously, this will make a HUGE DIFFERENCE in the texture of your cake. Sohla El-Waylly has a great video where she explains lots of cake chemistry, including the importance of room temperature ingredients here.