Preheat the oven to 350°F (175°C). Lightly grease a 9x9-inch baking pan or line it with parchment paper.
In a large bowl, use a whisk to combine the melted butter, 1/2 cup granulated sugar, and 1/4 cup brown sugar. Whisk until well combined.
Add2 large eggs and whisk until combined, then whisk in 1 (15 ounce can) pumpkin puree, and 2 tablespoons sour cream, and 1 1/2 teaspoons vanilla extract until the batter is smooth. Scrape down the sides and bottom of the bowl to ensure all ingredients are evenly mixed.
In a separate medium bowl, whisk together 1 1/4 cup all-purpose flour, 2 teaspoons pumpkin pie spice, 3/4 teaspoon baking powder, 3/4 teaspoon baking soda, and 1/2 teaspoon salt.
Gradually add the dry ingredients to the wet ingredients. Use a spatula to fold the mixture, stirring until just combined. Do not over-mix. Spread the batter evenly into the prepared baking pan.
Prepare the Streusel Topping: In a medium bowl, whisk together 1 cup all-purpose flour, 3/4 cup brown sugar, 2 tablespoons granulated sugar, and 1 teaspoon pumpkin pie spice. Pour in the melted butter and mix with a fork until large crumbs form. Sprinkle the streusel topping evenly over the cake batter.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool completely before adding the glaze.
Prepare the Glaze: In a small bowl, whisk together 1/2 cup powdered sugar and 1 tablespoon milk. Gradually add more milk, 2 teaspoons at a time, until the glaze is thin enough to drizzle. Drizzle the glaze over the cooled cake.