Potatoes Anna is basically a dish where you take any type of potato, slice it really thinly, then arrange the slices in a pan and bake. (It's a great holiday dish because you can rope in a family member to either be the slicer or the arranger!)
Preheat oven to 450 degrees F. Melt butter in a small saucepan over medium heat. Meanwhile, slice the sweet potatoes on a mandoline. Soak prunes in port for 20 minutes, then drain and roughly chop. Coat a cast-iron skillet with butter, then arrange half of the potatoes in an overlapping circle until the bottom of the skillet is covered. Brush the top generously with melted butter, and sprinkle with salt and pepper. Scatter half of the prunes over the sweet potatoes, then repeat the whole process. Press down on the potatoes so they are compact.
Bake 20–30 minutes until potatoes are cooked through and nicely caramelized. Run a knife around the edge of the skillet to loosen. Place a serving plate on top of the skillet and flip over to serve.