Bring a large pot of water to a rolling boil. Carefully add in eggs, one by one. When the water returns to a slight boil, set your timer for 8 minutes. In the meantime, fill a small bowl full of ice water.
Meanwhile, in a small mixing bowl, add kewpie mayo, rice wine vinegar, grainy mustard, dill, chives, and salt together. Mix until incorporated and set aside.
When the timer goes off, remove only two of eggs and place them in the ice bath. These will be your medium boiled eggs. Set another timer for 4 minutes and let the remaining eggs cook until timer goes off.
Crack and peel the skin off of the 6 hard boiled eggs but keep the medium boiled ones that you pulled out earlier to the side. Roughly chop the hardboiled eggs and place them in a bowl. Add the sauce mixture and season with salt and pepper and set aside.
Line up four pieces of white bread with the crust removed and smear some grainy mustard onto each one. Crack and peel the two medium boiled eggs and slice them down the middle lengthwise. Place each half of the egg on the mustard-smeared toast so that the narrow tips of the eggs are parallell to the longest side of the bread. (see picture above). Pile the egg salad all around the medium boiled egg so that it covers it.
Smear sriracha on the other slices of crustless bread and place them on top, sriracha-side down. Use a super sharp serrated knife and cut the sandwiches in half to show off the medium boiled egg in the center.