Preheat oven to 425F.

With a sharp knife to make 1/8-inch-thick slices in each potato, crosswise, without cutting through the bottom, so potato remains intact. Using two wooden spoons, place a wooden spoon lengthwise along both long sides of the potato. The sides of the spoon will prevent the knife blade from going too far down.

Arrange potatoes on baking sheet.

In small bowl combine butter, olive oil, panko, rosemary, kosher salt and pepper.

Brush potatoes generously with the butter mixture, using fingers to separate slices and brush mixture between slices also.

Roast 45 minutes or until potatoes are tender.