Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it with cooking spray.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Stir in the vanilla extract and sourdough starter until combined.
In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and ground cinnamon.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
If using, fold in the chopped nuts.
Divide the batter evenly among the muffin cups, filling each about two-thirds full.
In a small bowl, mix together the brown sugar, melted butter, and ground cinnamon for the topping.
Sprinkle this mixture generously over the batter in each muffin cup.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.