Build the Sandwich
Lay the bread slices on a clean work surface and spread 1 side of each slice with some of the Russian dressing. Layer 1/2 of the slices with some of the corned beef. Divide the sauerkraut evenly over the meat, then top the sauerkraut with the slices of cheese. Place the remaining bread slices on top, dressing sides down, and set aside.
Heat 2 heavy large skillets over medium heat. When hot, brush 1 side of each sandwich with some of the butter and place in the hot pans, buttered sides down, and cook until crispy and golden brown, about 4 minutes. Spread the unbuttered, top sides of the sandwiches with the remaining butter and turn the sandwiches to cook on the second side until golden brown, crispy, and the cheese is melted, 3 to 4 minutes longer.
Guinness Corned Beef
Remove corned beef brisket from packaging and rinse well. Add to slow cooker and sprinkle onto beef the spice packet from the package. Pour Guinness onto corned beef and then add water until brisket is completely covered.
Set to low for 8 hours.
In a bowl, combine all ingredients except salt and whisk until well-blended. Add salt to taste, cover and refrigerate until ready to serve.