Preheat oven to 350°F. Grease a 13 x 18 jelly roll pan with butter or spray with Baker's Joy.
In a large bowl, whisk together melted butter, milk, pumpkin and sugar. Whisk in eggs and vanilla extract. Then whisk in flour, baking soda, pumpkin pie spice and salt.
Pour batter into prepared jelly roll pan and bake for 20 minutes, or until a toothpick inserted in the center comes out clean.
Frosting:
In a large bowl cream butter and cream cheese until smooth. Beat in milk, vanilla extract and cinnamon on low speed until combined.
Then whisk in powered sugar - beating on high until frosting is smooth. Spread frosting on top of cake.
Refrigerate cake until ready to serve, and let cake rest on counter for 10 minutes before serving to let it warm up just a little.