Preheat oven to 350 degrees. Spray a 13 x 18 jelly roll pan with non-stick cooking spray.
In a large bowl, whisk together melted butter, milk, pumpkin and sugar. Add in eggs and vanilla extract and whisk together.
Add in flour, baking soda, pumpkin pie spice and salt and whisk until blended.
Pour batter into prepared jelly roll pan and bake at 350 degrees for 20 minutes , or until a toothpick inserted in the center comes out clean.
To prepare the frosting: Cream butter and cream cheese together in a bowl until smooth. Beat in milk, vanilla extract and cinnamon on low speed until combined.
Add in powered sugar a cup at a time and blend until smooth. Spread frosting on top of warm cake.
Refrigerate cake until ready to serve, and let sit for 10 minutes before serving. Serves 24.