- **Preheat the oven:** Preheat your oven to 425°F (218°C). Grease or line a 12-cup muffin tin with paper liners. 
- **Prepare the cinnamon swirl:** In a small bowl, mix softened butter, cinnamon, flour, and brown sugar until crumbly. Set aside. 
- **Make the muffin batter:** 
- - In a medium bowl, whisk together the flour, baking soda, and salt. 
- - In a large bowl, beat softened butter, sugar, and brown sugar until light and fluffy. Add the egg, vanilla, milk, and sour cream, mixing until smooth. 
- - Gradually fold in the dry ingredients until just combined (do not overmix). 
- **Prepare the streusel topping:** 
- - Mix the butter, flour, brown sugar, and optional nuts (if using) in a small bowl until the mixture is crumbly. Refrigerate until ready to use. 
- **Assemble the muffins:** 
- - Scoop batter into muffin cups, filling about 2/3 full. 
- - Add a small spoonful of the cinnamon swirl mixture to the center of each muffin and gently swirl it with a toothpick or knife. 
- - Top each muffin with the prepared streusel topping. 
- **Bake the muffins:** 
- - Bake at 425°F (218°C) for 5 minutes, then reduce the temperature to 350°F (177°C) and continue baking for 30-35 minutes, or until a toothpick inserted into the center comes out clean. 
- **Cool and serve:** Allow muffins to cool completely on a wire rack before serving.