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.
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Directions:
  1. Heat the oven to 375 degrees. Generously grease a 12-cup muffin tin.