Prepare a 9-inch square pan by lightly buttering it or lining with parchment paper.
In heavy 2 to 3-quart saucepan, melt butter over medium heat. Add sugar, cocoa powder, corn syrup and sweetened condensed milk, stirring constantly.
Cook over medium heat stirring frequently to prevent scorching until the temperature of the mixture reaches 248°F (firm ball stage.)
Remove from heat and stir in vanilla. Pour mixture evenly into the prepared pan.
When cool, turn candy out of pan and cut into ¾-inch squares.
Wrap individual pieces in 2¾ to 3-inch squares of nonstick aluminum foil or waxed paper.
Store in an airtight container.