Preheat the oven to 350°F. Grease a 9x13-inch baking pan or line it with parchment paper, leaving some overhang for easy removal.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, cinnamon, and nutmeg until evenly combined.
In a separate large bowl, whisk together the pumpkin purée, brown sugar, granulated sugar, eggs, vegetable oil, milk, and vanilla extract until smooth.
Gradually add the dry ingredients to the pumpkin mixture. Stir gently with a rubber spatula just until the ingredients are combined. Do not overmix.
To make the streusel, combine the flour, brown sugar, cinnamon, and salt in a medium bowl. Stir in the melted butter until coarse crumbs form, then fold in the chopped pecans.
Spread the pumpkin batter evenly into the prepared baking pan. Sprinkle the cinnamon pecan streusel evenly over the top.
Bake for 38–42 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs attached.
Remove the cake from the oven and let it cool in the pan for about 15–20 minutes. Transfer to a wire rack if desired and allow it to cool further before slicing.
For the glaze, whisk together the powdered sugar, milk, and vanilla extract until smooth. Drizzle over the cooled cake.
Slice and serve warm or at room temperature with coffee, tea, or your favorite fall beverage.