Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease with cooking spray.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
In a large bowl, beat the butter, granulated sugar, and brown sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Gently fold in the diced apples and chopped walnuts.
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