Preheat the oven to 350℉ and grease the 10x15 jellyroll pan or line with parchment paper.
Beat the eggs and add milk in a large bowl.
Add the melted butter and pumpkin to the eggs and milk. Continue to mix until all are combined.
Gradually add the spice cake mix, stirring until the batter is fully combined and no lumps remain.
Spread the batter in a greased and parchment paper lined 15x10x1 inch jelly roll pan and bake at 350 degrees for 18-20 minutes. When a toothpick comes out clean, the cake is done.
Spread frosting or other desired frosting toppings.
Cream Cheese Frosting
Cream the cream cheese, butter, and vanilla together
Slowly add the powdered sugar to the cream cheese mixture. When banana bars are cooled spread icing over top of the cake.
Notes
To use a Yellow or White Cake Mix: Add 2 teaspoons of pumpkin pie spice or a mix of cinnamon, nutmeg, and ginger to mimic the spice flavor.If you prefer homemade instead of boxed cake mix, you can substitute it with a homemade cake mix. Use 1 3/4 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, 1/2 teaspoon of baking soda, 1/4 teaspoon of salt, and 3/4 cup of granulated sugar.