Preheat your oven to 350°F (175°C). This bit is essential as it ensures your muffins bake evenly.

Grease your muffin tins or line them with paper liners. This keeps the muffins from sticking, making them easy to remove after baking.

In a large bowl, whisk together the eggs, sour cream, melted butter, and vanilla extract until well combined. This mixture will bring moisture to your muffins.

In another bowl, sift together the flour, sugar, baking powder, baking soda, salt, and ground cinnamon. Mixing these ingredients separately helps to combine them evenly.

Slowly add the dry mixture to the wet mixture. Stir delicately until just combined. Over-mixing can lead to tough muffins, so aim for a few lumps.

Add the diced tart apples to your batter. Fold them gently to ensure they’re evenly distributed without overworking the batter.

In a small bowl, combine the topping ingredients: granulated sugar, chopped walnuts, flour, and cinnamon. Cut in the chilled butter using a fork or pastry cutter until crumbly.

Spoon the batter into each muffin cup until they are about 2/3 full. This gives the muffins room to rise without overflowing.

Sprinkle the crunchy topping mixture generously over each muffin before placing them into the oven.

Bake for 20 to 25 minutes or until golden brown and a toothpick inserted in the center comes out clean.

Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack. Enjoy them warm or at room temperature!