PREP: Preheat the oven to 350 degrees. Lightly grease a 9X13-inch metal baking pan with nonstick cooking spray (or you can line with parchment or foil and then grease for easy cleanup).
DRY INGREDIENTS: In a large bowl, whisk together flour, cinnamon, pumpkin pie spice, nutmeg, baking soda, fine sea salt, and baking powder; set aside.
WET INGREDIENTS: In a medium bowl, add the oil (if using coconut oil, make sure it's melted and 100% cooled to room temperature), packed dark brown sugar, maple syrup, and heavy cream. Whisk until smooth. Add the vanilla and pumpkin. If the pumpkin is too wet, dab a few times with paper towel. I know 1 and 1/2 cups is ALMOST the whole can, but refrain from adding in more as it makes the cake too moist. Whisk until smooth.
COMBINE: Pour wet ingredients into dry. Mix until just combined, being careful not to overmix the batter. Spread the batter evenly in the prepared pan.
STREUSEL: in a medium bowl, whisk together the flour, brown sugar, cinnamon, and salt. Add the butter and mix with a pastry blender or your fingers until the streusel is evenly combined and forms small clumps. Sprinkle the streusel mixture over the cake batter.
BAKE: Bake until the edges begin to pull away from the sides of the pan and a toothpick inserted in the center comes out clean (or with a few moist crumbs attached) about 35 to 40 minutes (mine is always done at 35 minutes). Cool completely in the pan before icing.
GLAZE: Combine all the ingredients in a small bowl (start with 2 tablespoons milk and add more as needed). Whisk with a fork until smooth. Drizzle over the individual pieces and enjoy!