Fill a large pot about halfway with water and bring it to a boil on the stove. Salt the water generously. Add the potatoes in and let them boil for 5 minutes. Reduce the heat to a simmer and let them cook for another 10 minutes. They should just be fork tender. Drain them in a colander and then put the colander back over the pot off of the heat. Cover the colander with a clean towel and let the potatoes steam for another 15 minutes.

While the potatoes cook and steam, prepare the dressing. Wrap a bowl in a cold, wet towel. Then combine the egg yolk, vinegar, Dijon, dill, salt and pepper in the bowl and whisk it all together. Continue to whisk while you slowly pour in the canola oil until you have a pale, thick mayo. Then whisk in the whole grain mustard and buttermilk to finish it up and give it more flavor. Set the dressing aside.

When the potatoes are done and cool enough to handle cut them all in half, then cut each half into quarters. Add them to a large bowl along with the celery, scallions, dill, salt and pepper. Pour in the dressing and toss it all together thoroughly. Cover the bowl with plastic wrap and let the salad chill for 3 hours. Then serve it on the side of your meal and enjoy!