Toast 2 1/2 cups of pecans in a large skillet over medium-high heat, stirring occasionally, until they darken slightly and become fragrant, which should take just a few minutes.
Line two large cookie sheets with parchment paper, spray with cooking spray, and arrange the pecans into clusters of three, leaving about an inch between each cluster.
In a medium-sized saucepan, melt 1/2 cup butter over medium-high heat, then add 1 cup brown sugar, 1/2 cup light corn syrup, and 7 ounces sweetened condensed milk.
Increase the heat slightly and stir continually, cooking until the mixture reaches 235–240°F; remove from heat around 234°F to allow the residual heat to finish the caramel.
Stir in 1/2 teaspoon vanilla.
Quickly drizzle about 1 1/2 teaspoons of caramel over each pecan cluster, making sure the caramel touches all the nuts to stick them together.
In a microwave-safe bowl, melt 12 ounces of milk chocolate chips, heating in 30-second intervals and stirring between each.
Add 1/2 teaspoon shortening to the melted chocolate, let it sit for a minute or two, then stir until smooth.
Spoon about 1 1/2 teaspoons of melted chocolate over each cluster, swirling it gently with the back of your spoon.
Let the clusters cool and harden completely before serving or packaging.