In a small mixing bowl, add all the dressing ingredients and whisk until combined. Cover and store in the refrigerator until ready to use.
For the sandwiches
Heat a griddle or skillet over medium heat and melt the 1 tablespoon of butter.
To the pan, add the sliced corned beef and cook until the meat is hot. Transfer to a plate and set aside. Wipe clean the pan and heat over medium heat.
For each sandwich, butter one side of a slice of marble rye with the softened butter and place it on the heated pan. Place on two slices of Swiss cheese, 4 ounces of the slices and warmed corned beef, 1/4 cup of the sauerkraut, 2-3 tablespoons of thousand island dressing, 2 more slices of Swiss cheese, and top with another slice of buttered marble rye, butter side up.
Grill the sandwich on both sides for 3-5 minutes on each side or until golden brown. Repeat with the remaining sandwich ingredients.
Cut the sandwiches in half and serve immediately.
Notes
Reuben Sandwiches are best enjoyed fresh, but you can prepare some components in advance, such as the Thousand Island dressing. Assemble the sandwiches and grill them just before serving to maintain their texture and taste.
Leftover Thousand Island dressing can be stored in an airtight container in the refrigerator for up to one week.