Preheat oven to 400 degrees F. Microwave potatoes for 4-5 minutes, turning over once, until almost tender when inserting knife into center.
Meanwhile, stir together softened butter and ranch mix until smooth.
Using a towel or oven mitt to protect your hand, make 1/4″ slices in potatoes, stopping before you slice through to the bottom. (Placing potato in a large wooden spoon during this step can help prevent slicing through completely.)
Place sliced potatoes on baking sheet. Use a spreading knife to push some of the seasoned butter into each incision.
Place a piece of cheese into each incision (cut to just fit).
Bake potatoes for 10 minutes in preheated oven. Then, broil for 5-7 minutes, until skin starts to brown.