Preheat oven to 350 degrees and butter a 9-inch square glass baking dish.
Make streusel. In a small bowl, mix together 1/2 cup brown sugar, 1/3 cup flour, 1 1/2 teaspoons cinnamon, pecans, and melted butter. Set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, ginger, nutmeg, allspice, and salt. Set aside.
Place butter and brown sugar in a bowl for an electric mixer and beat until well blended.
Add eggs one at a time, scraping down sides of bowl between additions. Add vanilla extract.
Add pumpkin and sour cream and beat just until blended. Add flour mixture and beat just until incorporated.
Spread half of better in bottom of prepared baking dish. Batter will be thick. Sprinkle half of streusel on top. Pour remaining batter into pan and spread over streusel. Sprinkle remaining streusel on top.
Place in oven and bake for approximately 45 minutes, or until set.
For glaze, place brown sugar, heavy cream, and butter in a small saucepan. Heat over medium-low heat, stirring frequently, until sugar and butter are melted. Drizzle over coffee cake.