Combine sugar, corn syrup, half-and-half, heavy cream and salt in a large saucepan.

Stir mixture over low heat about 5 minutes, until sugar is dissolved.

Bring to a boil and boil for 1 minute.

Dip a pastry brush in warm water and brush the sides of the saucepan down. Remove saucepan from heat.

Stir in semisweet chocolate until mixture is smooth and chocolate has melted.

Brush down the sides of the saucepan again with the wet pastry brush and place the pan over medium heat. Cook, without stirring, until the temperature reaches 238 degrees F on a candy thermometer.

Remove pan from heat.

Add, without stirring, butter and vanilla.

Place the saucepan in a larger pan that contains cold water and cool the mixture to 110 degrees F.

When cool, stir it with a wooden spoon until it begins to lose its sheen.

Add remaining 1 tbsp. half-and-half and beat mixture until it is just blended.

Scrape down the sides of the pan with a rubber spatula. Let stand and stiffen for 5 minutes. Then, if necessary, adjust the consistency of the frosting by stirring in half-and-half 1 tsp. at a time until it is spreadable.

Use immediately or cover the surface of the frosting with plastic wrap.