Prepare the Dry Ingredients. In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and ground cinnamon. Ensure they're well combined.
Combine Wet Ingredients. In another bowl, beat the eggs, then add the buttermilk, melted butter, and vanilla extract. Mix until everything is thoroughly combined.
Combine Wet and Dry Mixtures. Pour the wet mixture into the dry ingredients and gently stir until just combined. Be careful not to overmix; a few lumps are okay.
Fold in Apples (and Optional Nuts/Raisins). Gently fold in the diced apples. If desired, add chopped nuts or raisins for extra flavor and texture.
Fill Muffin Cups. Line a muffin tin with paper liners or grease it lightly. Spoon the muffin batter into each cup, filling them about two-thirds full.
Bake. Preheat your oven to 350°F and bake the muffins for 18-20 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs attached.
Cool and Enjoy. Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.