Spread 1 tablespoon of softened butter evenly on one side of each slice of rye bread (these will be the outside surfaces for grilling). On the unbuttered inside of two slices, spread 1 teaspoon yellow mustard and 1 teaspoon Dijon mustard on each piece (adjust ratio if you prefer more of one).

Assemble sandwich

Place two slices of rye, mustard-side up, on a work surface. On each, arrange 4 ounces (half of the corned beef quantity per sandwich) of thinly sliced corned beef in an even layer. Divide and distribute drained sauerkraut (if using) evenly over the meat, then top each with two slices of Swiss cheese. Add two dill pickle slices per sandwich and sprinkle about 1/2 teaspoon freshly ground black pepper over the filling. Top with the remaining mustard-spread bread slices, butter-side out.

Grill the sandwich

Heat a heavy skillet, cast-iron pan, or griddle over medium-low to medium heat. Place sandwiches in the skillet and press down gently with a spatula or a heavy lid. Cook for 3–4 minutes per side, checking after 3 minutes—when the bottom is golden brown and crispy, carefully flip the sandwich. Continue cooking another 3–4 minutes until the bread is golden and the Swiss cheese is fully melted. If using a lid for faster melting, cover the pan for 1–2 minutes after flipping.

Rest and serve

Transfer sandwiches to a cutting board and let rest for 1 minute to allow the cheese to settle. Cut each sandwich in half diagonally. Serve hot with extra pickles and a side of Russian or Thousand Island dressing if desired.