Add the powdered sugar, dry milk, cocoa powder, and salt to a medium mixing bowl, whisk together well.
In a small glass bowl add the butter and water, melt on high for 30 seconds. If not fully melted, add 10 second bursts till it is.
With a metal spoon, slowly add in the melted butter mixture and vanilla, mixing well till combined.
Take a sandwich box and lightly spray with non-stick cooking spray or butter it. Scoop the mixture into the box and pat out flat using the spoon or clean fingers.
Cover tightly and let chill for 30 minutes or longer to set up. Carry in the box or pop out and wrap in plastic wrap or wax paper.