Preheat oven to 400 degrees.
Melt butter in medium saucepan over low heat.
Remove from heat.
Stir in sugar and cocoa powder and mix well.
Add eggs and stir mixture. Do not beat.
Stir in evaporated milk and vanilla.
Pour into pie crust and bake at 400 degrees for 10 minutes. Then reduce the oven temperature to 350 degrees and continue cookings for 20-25 minutes, until crust is brown.