01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or lightly grease each cup.
02 - Sift together all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg in a medium mixing bowl.
03 - In a large bowl, blend softened unsalted butter with granulated sugar using a mixer until pale and fluffy. Add eggs individually, beating well after each addition, then incorporate vanilla extract.
04 - Gently mix in sour cream and whole milk until just combined.
05 - Gradually add sifted dry ingredients to the wet mixture, mixing until nearly incorporated. Using a spatula, fold in the diced apples.
06 - Stir light brown sugar, ground cinnamon, and melted unsalted butter together in a small bowl until fully blended.
07 - Spoon muffin batter to fill cups halfway, add a small spoonful of cinnamon swirl mixture in the center, top with remaining batter, then add another spoonful of cinnamon mixture. Lightly swirl each muffin with a toothpick for a marbled effect.
08 - Transfer pan to the oven and bake for 20 to 22 minutes, or until a toothpick inserted in the center emerges clean. Allow muffins to cool in pan for 5 minutes, then move to a wire rack.
09 - In a microwave-safe bowl, combine soft caramels, heavy cream, and a pinch of salt. Microwave in 20-second intervals, stirring after each, until the caramels melt and the glaze is smooth.
10 - Drizzle caramel glaze over fully cooled muffins before serving.