buttermilk: leche cortada (cuajada) suero de leche, suero de manteca (from diferent diccionaries)
This is not a product often found in the DR. I will not be surprised to find out that we might be using the English term, since what we call "suero" does not look exactly to what in the US is sold as buttermilk. (Btw, buttermilk is not milk with added butter.)
When a recipe calls for buttermilk, normally people just add few drops of lemon juice or vinegar to regular milk.