Preheat oven and spray a 9×13 baking dish with pam baking spray.
Using a large saucepan, combine the butter, water, and cocoa powder over medium to high heat until the butter melts.
Using a large bowl, whisk together the flour, sugar, eggs, baking soda, pumpkin pie spice, sour cream, and vanilla until combined.
Gradually whisk in the wet ingredients until combined.
Pour batter into the baking dish and bake for 20-25 minutes or until a toothpick comes out clean.
Allow to cool for 15 minutes.