In a shallow baking pan, spread pecan halves in a thin layer. Bake in a 350 degree F oven for 5 to 10 minutes or until pecans are a light golden brown, stirring once or twice. Remove from oven. Set aside. Line baking sheets with parchment paper or greased foil.
For caramel mixture, in a 3-quart saucepan, combine sugar, 1-1/2 cups of the whipping cream, the corn syrup, butter, and salt. Cook and stir over medium-high heat to boiling.
Keeping mixture boiling, very slowly add remaining 1/2 cup whipping cream. Clip a candy thermometer to side of the pan. Reduce heat to medium-low; mixture should boil at a moderate, steady rate over entire surface.
Cook, stirring occasionally, until mixture reaches 244 degree F (firm-ball stage). Keep an eye on the mixture so it doesn't boil over. Remove from heat immediately. Remove candy thermometer. Pour into a 4-cup glass measuring cup.
For large candies, mound pecans in 20 groups, 3-1/2 inches apart on the baking sheets. For small candies, arrange 3 or 4 pecan halves 2 inches apart.
Quickly pour half of the caramel mixture over pecans. (If caramel is very hot, it will run more. Just push caramel back to pecan mound with a small spatula.) Repeat with remaining caramel mixture. (Caramel will have cooled enough to mound.)
In a small saucepan, melt the 1 cup semisweet chocolate pieces over low heat. In another small saucepan, melt the 1 cup vanilla baking pieces over low heat. Spread half of the caramel mounds with semisweet chocolate and the other half with vanilla baking pieces.
Using the same small saucepans, melt the 1/3 cup semisweet chocolate pieces and 1 teaspoon shortening in one saucepan and the 1/3 cup vanilla baking pieces and 1 teaspoon shortening in the other. Drizzle semisweet chocolate over vanilla-frosted candies; drizzle the melted vanilla baking pieces over the semisweet chocolate-frosted candies. Drag a toothpick through the drizzle, if you like. Let stand until set. Makes 18 to 20 large or 30 to 35 small candies.
Vanilla baking pieces are the same size and shape as the familiar semisweet chocolate pieces.