Step 1

In a medium saucepan over medium-low heat, stir together butter and chocolate, until chocolate is melted.

Step 2

Stir in powdered sugar and evaporated milk. Mixture will be very thin. Increase heat to slightly more than medium and bring mixture to a low boil. Cook for 4 to 5 minutes, stirring continuously, until thickened.

Step 3

Remove saucepan from heat and let cool for 5 to 10 minutes. It will thicken more, the longer it cools.