Lightly oil inside of 1 1/2 to 2 quart saucepan.
Combine chocolate, sugar, milk, corn syrup, and salt in saucepan.
Cook over medium heat, stirring constantly, until sugar dissolves and mixture comes to boil.
Wash down crystals on inside of pan above liquid line, using clean pastry brush dipped in water.
If using candy thermometer, clip it onto pan; allow mixture to cook until thermometer registers 238°F (soft-ball stage).
If not using candy thermometer, test for soft-ball stage (syrup forms soft ball in cold water).
Do not stir while mixture is cooking; when it reaches proper temperature, remove from heat immediately.
Add butter, but do not stir.
Cool to 110°F.
If not using thermometer, cool until bottom of pan feels lukewarm.
Add vanilla and nutmeg.
Start beating vigorously; beat until mixture loses its glossy look.
Pour into well-oiled 8 x 8 x 2-inch pan.
Cut into squares when cool.
Store in airtight container in cool place.
Let stand day or two to allow flavor to develop.