Preheat the oven to 350F and grease a 9-inch square or round baking pan. Whisk the flour, brown sugar, cinnamon, and salt in a bowl.
Cut in the cold cubed butter with a fork or pastry cutter until the mixture forms coarse, pea-sized crumbs, then stir in the pecans if using.
Mix the Batter
In a separate bowl, whisk the melted butter, brown sugar, eggs, pumpkin, milk, and vanilla until smooth.
Add the flour, cinnamon, pumpkin pie spice, baking soda, baking powder, and salt. Stir just until combined with no dry streaks, being careful not to overmix.
Layer and Bake
Spread the batter into the pan and scatter the streusel evenly over the top.
Bake for 35 to 45 minutes, until a toothpick in the center comes out with a few moist crumbs. Cool in the pan before glazing.
Glaze
Whisk the powdered sugar, maple syrup, and enough milk to make a thin, pourable glaze. Drizzle over the cooled cake.
Notes
Cold butter for streusel: Cold butter cut into the flour gives real crumbs, softened butter turns it into a paste.
Don't overmix: Stir just until the dry streaks disappear for a tender crumb.
Cool before glazing: A warm cake melts the glaze into a puddle.