It happens to the best of us. You realize that you are missing an ingredient in a recipe.
Yes, everyone knows that you should read the recipe all the way through before you start it. Okay, so you didn't. Now what?
You are not really supposed to make too many adjustments when baking but there are some substitutions that you can get away with.

Here is my list:

Instead of 1 cup of buttermilk, you can use 1 tablespoon of lemon juice or vinegar plus enough milk to make a cup.

Instead of self rising Flour, you can use 1 cup all-purpose flour, plus
1 1/4 teaspoons baking powder and a pinch of salt.

Instead of 1 cup of heavy cream when baking (not whipping) you can use
3/4 of a cup of whole milk and 1/4 of a cup of butter.

Instead of 1/2 a cup of brown sugar, you can use 1/2 a cup of sugar mixed with 2 tablespoons of molasses

Instead of sour cream, you can use full fat yogurt (the texture might change slightly)

Instead of vanilla extract, you can use almond or some other kind of extract.