Preheat the oven to 350 degrees F and lightly grease a 9x13 inch cake pan.
Dry ingredients: In a large mixing bowl, whisk together the flour, baking powder, salt, pumpkin pie spice and sugar until well combined.
Wet ingredients: In a medium bowl, whisk together the pumpkin puree, oil, milk and vanilla until well combined.
Add the wet ingredients to the dry bowl and stir with a large spoon until just combined. Be careful not to over mix, or the cake will be dense.
Pour the cake batter into the prepared pan. Set aside while you make the streusel.
Streusel: In a medium bowl, mix together the melted vegan butter, flour, brown sugar, cinnamon and pumpkin pie spice with a fork until well combined. Use your hands if needed; it will be crumbly.
Sprinkle the streusel topping all over the cake batter, place in the oven and bake for 40-45 minutes, or until a toothpick comes out clean in the middle.
If using optional glaze: Stir all ingredients together in a small bowl, and drizzle glaze over cooled coffee cake. Serve with a cup of coffee and enjoy.
Storage: The coffee cake can be store at room temperature, covered well, for about 2 days. Keep it in the refrigerator for up to 5 days. It can also be frozen if needed, but it's best served the same day.