Mix butter, sugar, cocoa, and evaporated milk in a sauce pan. Bring to a boil, and boil until ready. Last time I heated it to 230° F, which was about right. I left the burner on medium while it boiled so the temperature went up slowly.
Pour into a blender and blend for 2 to 4 minutes. Add the vanilla while the blender is running.
I tried replacing 1⁄2 cup sugar with 3⁄8 cup corn syrup to prevent crystallization when the sauce cools. The leftover sauce is in the fridge now. If it works okay, I'll probably try with less corn syrup next time, maybe replacing 1⁄4 cup with 3⁄16 cup.