One of the biggest mistakes people make with custard pies, including buttermilk pie, is overwhipping the fillings. The mantra here is “gentle, yet assertive.” This is because you’re working with whole eggs, which can scramble and split if they become overwhipped. After all, you’re not trying to make poofy meringues with your egg whites. 

Besides making sure to not overwhip your ingredients, you’ll also need to be sure that you pay attention to the order in which you’re adding them to the pie. Most buttermilk pie recipes start with mixing together the butter, eggs, and flour before adding the buttermilk and lemon juice. This is because the acidic ingredients can curdle the eggs if added prematurely and agitated by the stirring. You’ll only want to mix your acids in enough so that they are completely mixed into the filling, but not so much that chunks of egg start to surface.