01 -  Combine granulated sugar, brown sugar, flour, and cinnamon in a medium bowl. Stir in melted butter until mixture forms coarse crumbs; set aside. 
 02 -  In a large bowl, whisk together flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg until evenly blended. 
 03 -  Using a stand mixer or handheld mixer, beat softened butter on medium speed until creamy. Add eggs one at a time, then add vanilla extract, mixing until incorporated. 
 04 -  Add pumpkin puree and milk to the butter mixture, blending until well combined. Scrape down sides as needed to ensure uniform texture. 
 05 -  Gradually fold the dry ingredients into the wet mixture just until no streaks remain; avoid overmixing to preserve cake tenderness. 
 06 -  Pour batter into a greased 9x13 inch baking dish, smoothing the top. Evenly sprinkle streusel mixture over the batter. 
 07 -  Place dish in a preheated 175°C (350°F) oven and bake for 35-40 minutes, or until a toothpick inserted into the center yields moist crumbs. 
 08 -  Mix powdered sugar, milk, and vanilla extract in a small bowl until smooth. If desired consistency is thinner, add milk in small increments. 
 09 -  Drizzle glaze over cooled coffee cake, slice, and serve.