Line a 9 x 13-inch glass baking dish with waxed paper and set aside. Let the waxed paper overlap the sides of the dish so you can pull it out the mixture when it is firm.
Mix peanut butter, melted butter, vanilla, brown sugar and salt in a large bowl until combined.
Add powdered sugar 1 cup at a time until combined.
Spread mixture in prepared glass baking dish and smooth out with a rubber spatula or press down with your hands until it is uniform throughout. Freeze for 30 minutes.
Line a baking sheet with waxed paper and set aside.
Remove dish from freezer and lift out peanut butter mixture onto a flat surface. Using a bunny-shaped cookie cutter, cut out bunny shapes and place on prepared baking sheet. After rolling out all the shapes, use the leftover mixture and roll between two sheets of waxed paper and cut out more bunnies until mixture is used up completely.
Freeze bunnies for one hour until completely firm and easy to handle to dip into chocolate.
Line another baking sheet with waxed paper and set aside.
Melt milk chocolate and shortening in the microwave in 25-second increments until completely melted. Removing 3 bunnies from the freezer at a time, dip into chocolate resting the bunny on a fork. Make sure both sides of bunny are covered with chocolate and place on a baking sheet lined with waxed paper. Refrigerate for 30 minutes or until chocolate is firm.
Prep time does not include refrigeration of bunnies.
Peanut Butter Bunnies can be stored in the refrigerator for up to 2 weeks or frozen for up to 2 months.