Preheat the Oven:

Heat oven to 350°F (175°C). Grease and flour an 8x8-inch square baking pan or line it with parchment paper for easy removal.

Make the Streusel:

In a small bowl, mix brown sugar, flour, and cinnamon. Cut in the cold butter with a fork or your fingers until the mixture looks like coarse crumbs. Stir in nuts if using. Set aside.

Cream the Butter & Sugar:

In a large bowl, beat the softened butter with both sugars until light and fluffy—about 2–3 minutes.

Add Wet Ingredients:

Mix in the eggs one at a time. Then stir in pumpkin, sour cream, and vanilla until smooth.

Mix Dry Ingredients:

In a separate bowl, whisk together flour, baking powder, baking soda, spices, and salt.

Combine the Mixtures:

Gradually add the dry ingredients to the wet, mixing just until combined. Don’t overmix.

Layer the Cake:

Pour half the batter into the prepared pan. Sprinkle half the streusel over it. Repeat with remaining batter and top with the rest of the streusel.

Bake:

Bake for 45–50 minutes, or until a toothpick inserted into the center comes out clean and the top is golden.

Cool & Serve:

Let cool in the pan for at least 20 minutes before slicing. Enjoy warm, or let it cool completely and dust with powdered sugar before serving.