To make the dressing, combine all of the dressing ingredients in a bowl, whisk until very well combined. Set aside.
Spread ½ tablespoon of butter on each slice of bread.
Spread dressing on the other sides of each slice of bread, and top half of them with corned beef, cheese, and sauerkraut. Top with another slice of bread, buttered side up.
Heat a skillet over low-medium heat, arrange the sandwiches (2 at a time) buttered side down, cook for 3 minutes while gently pressing down with a spatula, flip, then cook for 3 more minutes or until the bread is lightly toasted and crispy.
Serve warm with potato chips.
Make a “Rachel”. A Rachel Sandwich is similar to a Reuben, except the corned beef is replaced with turkey (or sometimes pastrami) and the sauerkraut is replaced with cole slaw or shredded cabbage.
Make it extra cheesy. Put a slice of cheese on both the bottom and top of the sandwich for an extra melty Reuben sandwich.