Spray a 15x10x1-inch sheet pan with baking spray. Preheat oevn to 325 degrees.
Using an electric mixer, cream butter, cream cheese, and sugar until light and fluffy.
Add eggs, one at a time, scraping down the sides of the bowl to make sure everything is evenly mixed.
Beat in vanilla.
Add flour and beat until blended.
Pour batter into prepared baking dish. Spread it evenly. Bake for 30 to 35 minutes. Let cool.
To make icing, stir together sugar and evaporated milk in a medium saucepan. Bring to a boil over medium heat. Cover and turn heat down slightly so the mixture doesn't boil over. Let simmer with lid on for 2 minutes.
Remove from heat and add butter and chocolate chips. Stir until melted. Let icing cool slightly and then pour evenly over cake.
Sprinkle with sprinkles if desired.