There's nothing better than rich and creamy European style cultured butter!
Mix buttermilk and heavy cream in a glass jar. Cover loosely with a towel and let sit in a warm place for 12-36 hours. This cultures the butter. When the cream has thickened and tastes a bit tangy, it's ready to churn.
Beat cream until it forms stiff peaks. I like to use my stand mixer but a hand-held mixer works just fine. Reduce to low and then stop when the cream separates.
Drain off as much of the liquid as possible. Mix ice with water, then pour 1/2 cup of the cold water over the butter solids. Use a spatula (or just use your hand) and knead the butter in the water against the side of the bowl. Drain the liquid and repeat the process with another 1/2 cup of water. Repeat until the water remains clear. Knead one last time to get as much water out of the butter as possible. Washing the butter in this way prevents spoilage.
If you'd like salted butter, mix in salt to taste. Store in an air tight container. If you're not going use the butter in about a week, freeze it.
Based on this recipe.