Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line it with parchment paper.

Prepare the Cake Batter: In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, cinnamon, and nutmeg.

In another bowl, cream the butter until light and fluffy. Beat in the pumpkin puree, eggs, and vanilla extract. Mix in the sour cream until well combined.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

In a small bowl, combine flour, brown sugar, cinnamon, and cold butter. Use a pastry cutter or your fingers to work the butter into the mixture until it resembles coarse crumbs.

Stir in the chopped pecans if using.

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

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Cool and Glaze (Optional):

Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

If desired, whisk together the powdered sugar, milk, and vanilla extract to make a glaze. Drizzle over the cooled cake.