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

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, ginger, and nutmeg.

In a large bowl, beat the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs one at a time, beating well after each addition. Mix in the pumpkin purée, sour cream, and vanilla until smooth.

Add the dry ingredients to the wet ingredients and stir just until combined. Do not overmix.

In a separate bowl, make the streusel by combining the flour, brown sugar, cinnamon, and salt. Cut in the cold butter with a pastry cutter or fork until the mixture resembles coarse crumbs.

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

Bake for 35 to 45 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Let the coffee cake cool in the pan for at least 20 minutes before slicing.

If using the glaze, whisk together the powdered sugar, milk, and vanilla until smooth, then drizzle over the cooled cake before serving.