While I always advocate using the exact ingredients that are called for in a recipe, you may find these substitutions helpful if you are in a pinch…

Butter
½ cup unsalted butter = ½ cup salted butter + reduce salt in recipe by ¼ teaspoon

Buttermilk
1 cup buttermilk = 1 tablespoon lemon juice or white vinegar + enough milk to make 1 cup. Let stand for 5 minutes
1 cup buttermilk = 1 cup plain yogurt

Cake Flour
1 cup cake flour =  ¾ cup sifted all-purpose flour + 2 tablespoons cornstarch

Chocolate & Cocoa Powder
1 ounce unsweetened chocolate = 3 tablespoons natural unsweetened cocoa powder + 1 tablespoon unsalted butter, vegetable oil or shortening
3 tablespoons cocoa powder (natural unsweetened) = 1 ounce unsweetened chocolate + reduce fat in recipe by 1 tablespoon
3 tablespoons cocoa powder (Dutch process) = 3 tablespoons natural unsweetened cocoa powder + 1/8 teaspoon baking soda

Half-and-Half
1 cup half-and-half = ½ cup whole milk + ½ cup heavy cream

Pumpkin Pie Spice
1 teaspoon pumpkin pie spice = ½ teaspoon ground cinnamon + ¼ teaspoon ground ginger + 1/8 teaspoon ground allspice + 1/8 teaspoon ground nutmeg

Sour Cream
1 cup sour cream = 1 cup plain yogurt

Vanilla Beans
1 vanilla bean = 2½ teaspoons vanilla extract