Prepare the Cake Batter

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Grease and flour a 9-inch round or square cake pan, or line it with parchment paper for easy removal.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, ginger, and salt. Set aside.
3. Cream the Butter and Sugar: In a large bowl, beat the softened butter and granulated sugar together until light and fluffy. This should take about 3-4 minutes.
4. Add the Eggs and Pumpkin: Add the eggs one at a time, beating well after each addition. Then, stir in the pumpkin puree and vanilla extract until combined.
5. Combine with Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients, mixing until just combined. Be careful not to overmix.

Make the Streusel Topping

1. Mix Streusel Ingredients: In a separate bowl, combine the brown sugar, flour, cinnamon, and melted butter. Stir in the chopped nuts if you’re using them. The mixture should be crumbly.

Assemble and Bake the Coffee Cake

1. Layer the Batter: Pour half of the batter into the prepared cake pan, spreading it out evenly with a spatula.
2. Add Streusel: Sprinkle half of the streusel topping over the batter.
3. Add the Remaining Batter: Add the remaining batter on top of the streusel and spread it out evenly.
4. Top with Streusel: Sprinkle the remaining streusel on top of the cake.
5. Bake: Bake in the preheated oven for 50-55 minutes, or until a toothpick inserted into the center of the cake comes out clean (a few crumbs are okay).
6. Cool: Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.