Make the streusel & filling: In a small bowl, combine the flour, sugar, pumpkin pie spice, and salt.

Cut in the butter: Cut in the cold butter until small crumbs form (you can use your hands, a pastry cutter, two forks, or a food processor). Place in the fridge.

Preheat and prep: preheat the oven to 350° and line a 9x9 inch pan with parchment paper.

Dry ingredients: whisk together the flour, sugars, baking powder, baking soda, salt, and pumpkin pie spice.

Wet ingredients: In another bowl, whisk together the oil, egg, pumpkin, sour cream, milk, and vanilla.

Combine: Add the wet ingredients into the dry, and fold just until combined. The batter will be thick!

Assemble: spoon half of the batter into the prepared pan, carefully spreading out evenly. Add about half of the crumble filling on top of that, and then the remaining batter. This part can be a little tricky because the streusel likes to stick to the batter; I usually use a small offset spatula to spread it evenly over the top. Scatter the remaining streusel on top.

Bake: Bake for ~35-45 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Start Timer(Cover with foil if tops are browning too quickly). Cool in the pan for ~10-15 minutes while you make the glaze, then use the overhang of the parchment to lift up and out of the pan. Start Timer

Glaze: whisk together the powdered sugar, maple syrup, and milk until smooth. Drizzle over the cake (it can still be slightly warm, mmm)

Serve: cut into squares and serve warm or at room temperature.