Line a 9x9” pan with foil and spray with cooking spray.
Place brown sugar, cream, and butter in a saucepan over medium-high heat. Stir until melted. Bring to a rolling boil, then reduce heat to low and place a candy thermometer in the pot.
Stir occasionally and simmer until the mixture reaches 236º, about 7-8 minutes. Turn off heat and stir in vanilla and salt. Whisk in powdered sugar. Pour into prepared pan and allow to set at room temperature (a few hours, but this can be done the day ahead).
When ready to cut, remove from pan. Use a large kitchen knife and cut the square into quarters. From here, use small cuts to cut them into small squares.
Be sure to use small but swift cuts. The candy is fragile and is prone to flaking and breaking, so a quick cut will make the candy stay together.
Melt chocolate according to package directions and dip each square, tapping off excess. Place on a wax paper lined cookie sheet and chill to set.