Grease an 8-inch square pan.
In a small saucepan, stir together peanut butter, vanilla, and salt.
Warm mixture over very low heat, being very careful not to burn anything to the bottom. Set aside, keeping it warm.
In a medium saucepan, stir together sugar, corn syrup and water.
Cook sugar mixture over medium-high heat just until it reaches 290 degrees, under hard crack. At first, the temperature will rise steadily, and then it will hover around 220 for what seems like an eternity. Be patient and keep stirring! Once it starts rising again, it will do so quickly.
Quickly stir in peanut butter until smooth.
Pour into prepared pan, working fast as the mixture begins to harden quickly!
Let cool on a wire rack for about 8 minutes.
Meanwhile, in a double boiler, melt the chocolate chips, stirring frequently until smooth.
Stir in shortening until completely melted and incorporated.
Pour chocolate over warm peanut butter base and use a spatula to spread, making sure to coat peanut butter evenly.
Allow to cool fully on wire rack (at least a couple of hours) before cutting with a sharp knife.