Line a 9 x 9-inch pan with foil.
Lightly coat the foil-lined pan with nonstick cooking spray.
Place the packed brown sugar, heavy whipping cream, and unsalted butter in a saucepan.
Set the saucepan over medium-high heat.
Stir the mixture until the butter is melted and the ingredients are fully combined.
Bring the mixture to a rolling boil.
Once the mixture reaches a rolling boil, reduce the heat to low.
Place a candy thermometer in the saucepan.
Simmer the mixture, stirring occasionally, until it reaches 236°F. This should take approximately 7 to 8 minutes.
Remove the saucepan from the heat.
Stir in the vanilla extract and salt.
Add the powdered sugar.
Whisk until the powdered sugar is fully incorporated and the mixture is smooth.
Pour the butterscotch mixture into the prepared pan.
Spread the mixture evenly in the pan.
Allow the filling to set at room temperature for several hours, or prepare it one day in advance and allow it to set overnight.
Once the filling is firm, lift it from the pan using the foil.
Place the set butterscotch square on a cutting board.
Use a large kitchen knife to cut the square into quarters.
Cut each quarter into approximately 16 small squares, yielding about 64 pieces total.
Melt the chocolate melts according to the package directions.
Stir the melted chocolate until smooth.
Dip each butterscotch square into the melted chocolate.
Tap off any excess chocolate.
Place each dipped square on a wax paper-lined cookie sheet.
Transfer the dipped candies to the refrigerator.
Chill until the chocolate coating is fully set.
Serve once set, or store as directed.