Preheat the oven to 350°F. Grease and flour a 9-inch round cake pan. In a large mixing bowl, cream together butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract and pumpkin puree. Gently fold in the sourdough starter.

In a separate bowl, whisk together flour, baking powder, baking soda, salt, and pumpkin spice. Gradually add to the wet ingredients and mix until just combined.

In a small bowl, mix flour, brown sugar, and cinnamon. Cut in the butter until the mixture resembles coarse crumbs. Set aside.

Pour the batter into the prepared cake pan. Sprinkle the streusel topping evenly over the batter.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Slice the cake and serve it with a cup of coffee or tea. For an extra treat, add a scoop of vanilla ice cream or a dollop of whipped cream on the side.

Wrap leftover cake tightly in plastic wrap or store in an airtight container. Keep at room temperature for up to 3 days or refrigerate for up to a week.