• Preheat the oven to 375 degrees and generously grease a bundt pan with non-stick cooking spray.
• Add the butter, water, oil, and cocoa to a saucepan and bring to a boil. Then, remove from heat.
• In a mixing bowl, add the flour, sugar, salt, and baking soda. Stir to combine.
• Carefully pour the chocolate mixture into the dry ingredients and mix well.
• Then, stir in the pumpkin puree, eggs, pumpkin spice, and vanilla extract.
• Pour the batter into the prepared bundt pan and bake for 50-55 minutes, or until completely cooked through.
• Allow the cake to cool completely before removing it from the bundt pan.

For the frosting:

• Add the butter, milk, sugar, cocoa powder, pumpkin spice, and vanilla extract to a small pot.
• Stir continuously over low heat until the butter has melted and the sugar has dissolved completely.
• Allow the frosting to cool slightly to drizzle over the cake.
• If you want a thicker frosting, all the frosting to cool completely. Then, frost the cake.
• Once the cake is frosted, add chopped walnuts on top, if desired.
• Cut into slices and serve.