01 - Set oven to 350°F and line a 12-cup muffin tin with paper liners.
02 - Whisk flour, baking powder, baking soda, salt, and cinnamon in a large bowl until evenly distributed.
03 - In a separate bowl, whisk eggs, brown sugar, melted butter, milk, and vanilla extract until fully incorporated.
04 - Add wet ingredients to the flour mixture, stirring gently until just combined. Do not overmix.
05 - Carefully fold diced apples and chopped walnuts into the batter.
06 - Spoon batter evenly into lined muffin cups, filling each about 3/4 full.
07 - Place pan in the preheated oven and bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean.
08 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.