3 apples (about 1 1/2 pounds), peeled, cored and sliced 1/4-inch thick
1/2 cup dark brown sugar
8 tablespoons (1 stick) unsalted butter
Pinch kosher salt
1/2 cup chopped walnuts, toasted (optional)

FOR THE MUFFINS
2 cups all-purpose flour
3/4 cup dark brown sugar
1 tablespoon baking powder
1/2 teaspoon ground cinnamon
1/4 teaspoon kosher salt
8 tablespoons (1 stick) unsalted butter, melted
2 large eggs, lightly beaten
3/4 cup sour cream
1 teaspoon vanilla extract.