Preheat oven to 400°.

Line a rimmed baking sheet with a Silpat, use a nonstick baking sheet or a medium-sized baking dish. ( I used an iron skillet as I had seen in one of the blogs.)

To make clarified butter put one stick butter in saucepan and slowly heat. As it heats the milk solids will rise to the top. Use a spoon and skim this off and discard. The milk fats in butter is what causes it to burn so quickly when cooking.

Slice sides off potatoes to make rectangled shaped potatoes. Cut sweet Potatoes and white potatoes into same size logs. Using a mandoline or very good knife skills, cut potatoes into 1/8" slices. Then cut them in half again or leave in long strips. Hold the stack of potato slices in the palm of one hand and use the other to shape them back into a brick, as you would a deck of cards. Some recipes I saw on line stacked them up in a pan to bake. I put about 6 potato slices to a stack.

Dot the potatoes with some of the clarified butter. I also tried using some duck fat on some of the stacks. Sprinkle with chopped rosemary and insert a few bay leaves here and there. Give a good sprinkle with salt and pepper.  Potatoes fan them out like fallen dominoes Top with clarified butter (clarified butter will not burn)

Bake for 30-40 minutes in oven until browned on edges and the tender is middle when tested with pick. Serve immediately.