I love to bake but I don't like wasting money or ingredients so I am happy to say that with some google magic I was able to find solutions to these problems. You may know about these but I had no idea and felt bad for buying buttermilk everytime I needed it and not using it all. Cake flour is another expensive ingredient that I felt wasn't worth the cost for the handful of recipes that I have come across that use it.
Here are some easy substitutions for ingredients that you probably already have on your cupboard:
Buttermilk - add 1 Tablespoon of vinegar to a measuring cup and then add milk up to the 1 cup mark. Let stand for 5-10 minutes
Cake Flour - 1 cup of AP flour minus 2 Tablespoons
Brown Sugar - add molasses to sugar and you have brown sugar. 1 cup sugar with 1 Tablespoon of molasses. For dark brown sugar, add another Tablespon
Oil - if you have run out of oil or just don't want to use it, you can use applesauce instead
Baking Chocolate (unsweetened) - I don't care to have stacks of partial baking squares sitting around the kitchen so I was very happy to find this substitution. 3 Tablespoons of cocoa powder with 1Tablespoon of melted butter equals 1 baking chocolate square
Sour Cream/Yogurt - these can be used interchangeably
I am sure there are more but these are the ones I commonly use in my baking adventures and hopefully now you will too.