On the baking sheets, arrange the pecan halves in snowflake-shaped clusters of 5 pecans each, overlapping the nuts in the center, leaving 2 inches of space between each.
In a medium saucepan, combine 1/2 cup of the evaporated milk, the corn syrup, and sugar and bring to a boil. Add the remaining 1/2 cup evaporated milk and the 4 tablespoons butter, and cook, stirring constantly, until it reaches the soft ball stage, 240 degrees F. Stir in the vanilla and remove from the heat. Let cool to 200 degrees F.
Using a tablespoon, spoon caramel on top of each nut cluster and let harden. (If the caramel becomes too stiff to pour, return to low heat, and cook, stirring constantly, until it returns to the proper consistency.)
To make the chocolate coating, in the top of a double boiler or in a metal bowl set over barely simmering water, combine 6 ounces of the chocolate and the shortening. Melt over low heat, stirring, until it reaches 116 to 118 degrees F, or feels fairly warm but not hot to the touch.
Remove from the heat and stir in the remaining 2 ounces of chocolate. Continue stirring until the chocolate reaches 80 degrees F or feels cool to the touch. Return to low heat and cook, stirring, until the temperature rises to 85 to 87 degrees F, or feels barely cool. Remove from the heat.
Drizzle 1 tablespoon of tempered chocolate over each cluster. Set aside in a cool place to harden. Store in an airtight container at room temperature for up to 2 weeks, with waxed paper separating the layers to prevent sticking.