Using a wooden spoon as a cradle, place each potato in the spoon and make several parallel slits into each potato top making sure not to slice completely through. Place 3 garlic slices between slits at the crown of each potato. Toss in a medium bowl with butter and olive oil. Place on a baking sheet and sprinkle generously with salt and pepper. Bake until tops are crispy and potatoes are cooked through, about 1 hour. Transfer to a platter and top with Herbed Sour Cream.
Herbed Sour Cream:
Combine ingredients in a small bowl. Season, to taste, and refrigerate until use.