I have used buttermilk to make French toast when I was out of regular milk. Just add a bit of sugar to the mix.
Here’s a dedicated Buttermilk Cookbook.
Use buttermilk in a variety of salad dressings not just “ranch” dressing. If you are really hard core (like my dear departed mother), drink a little ice cold buttermilk on a steamy hot day. Great thirst quencher. If not so hard core, I’ve seen it as an ingredient in some smoothie recipes.
I prefer to mix milk and lemon juice. Specifically, I use a tablespoon of lemon juice per cup of milk, mix it, and let it stand for at least 10 minutes. It’s not as strong as buttermilk, but it’s a decent approximation.
I found a recipe for drop biscuits that taste reminiscent of KFC biscuits, except better. I got it from America’s Test Kitchen, and I don’t have it with me, but I can post it later. The secret is to pour the melted butter into the cold buttermilk and let it clump into chunks before mixing with the dry ingredients. It is DIVINE.
I do love Popeye’s when I’m in the mood for biscuits that are sweeter. Mmmm-mmm. :d
Take your choice of buttermilk pie recipes.
Mix buttermilk with mango puree to make delicious mango lassi. Proportions are to taste, but 1/3 pulp to 2/3 buttermilk is about right. Tangy, sour, sweet, creamy, fruity and delicious. Heck, I should pick up some buttermilk on my way home tonight.
Fried chicken.
Make Koldskål! (lit. “cold dish”) - a very popular Danish summer dish. It’s easy to make and absolutely delicious:
This is the simplest recipe I can think of
Whisk two egg yolks with 2-4 large tablespoons of sugar (to taste, basically) + the seeds from one vanilla pod.
Mix with 1 litre of buttermilk and the juice and grated peel of a lemon.
Chill and serve cold.
Every family has their own recipe, and this basic version can be expanded in several ways. Some add 200ml of cream, some add yoghurt, some vanilla ice. You can also whisk a few egg whites and mix in. Or blended fruit. Or…
It is traditionally served with kammerjunkere, a small, crispy and sweet biscuit.
Koldskål is absolutely wonderful on a hot day. And it doesn’t really taste of buttermilk - most people I know loathe buttermilk and love koldskål, so give it a go.
Buttermilk is something quite new to me. A couple of weeks ago I bought some to make pancakes. They were very yummy but I ended up with left over buttermilk and no plan.
I tried Nigella’s buttermilk chicken and it was awesome!
Basically marinade chicken (she uses a whole chicken, I used thighs) in buttermilk, garlic, oil (she says “flavourless oil”) I used olive oill, salt, pepper, fresh rosemary and wee bit of maple syrup.
She marinades for “up to 2 days”. I marinaded for about 5 hours. Then roast.
It was DELISH!
Drink it! Cold buttermilk is awesome on a hot day.
EWWWW It might be just be me but it looks like vomit! I had enough of an issue marinading chicken in it but drinking it…sorry.
Try it anyway. It is very refreshing. Or make koldskål as I suggested, and drink that instead. No reason to waste good buttermilk.
It pretty much looks like a glass of milk or a yogurt drink.
I drink it, too. I don’t see what the ewww-ness is. It’s like a nutty, not so tart liquid yogurt. Good stuff, especially when very ice-cold.
Perhaps New Zealand Buttermilk is odd BUT it looked NOTHING like yoghurt or milk…well it kind of looked like milk you left in the sun for a while…milky-ish but lumpy-ish.
The pancakes were awesome and it did very good things to the chicken BUT I’m not drinking lumpy milk 
You can also buttermilk in scones! Scones are really simple to make, too.
And lemon bars, yum!
This chocolate cake is tasty, and it uses buttermilk.
Also, buttermilk is common in red velvet cake - but you could mix it up and make red velvet cheesecake.
Southern Jam Cake
3 cups sifted flour
1 teaspoon soda
1/4 teaspoon salt
1 teaspoon ground allspice
1 teaspoon ground cloves
1 teaspoon ground cinnamon
1 teaspoon ground nutmeg
1 cup chopped pecans
1 cup raisins or chopped dates, optional
1 cup butter
1 1/2 cups sugar
3 eggs, beaten
1 cup blackberry jam
1 cup buttermilk
Pour into three 9 in. layer cake pans, bake at 350 for 20 -25 mins. Frost with brown sugar frosting.
Sounds interesting! Do you melt the butter, or cream it with the sugar? I might try this one (without nuts, due to allergies), with some strawberry jam I’ve got in the freezer.
Another vote for marinating chicken in it: buttermilk, Tabasco, and minced garlic.
Dredge it in a mix of flour, cornmeal, paprika, and salt & pepper, then fry.