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.