Preheat the oven to 350°F. Place the pie crust into a 9-inch pie plate, crimping the edges as desired, and press the crust firmly against the sides of the plate to secure it. In the bowl of an electric mixer fitted with a paddle attachment—or using a hand mixer—cream together the brown sugar and softened butter until smooth. Add the egg and vanilla extract, beating until fully combined. Mix in the baking powder, salt, and flour until just incorporated. If using a stand mixer, add the candy bars to the bowl and mix on medium-high speed to chop them up—the mixer’s power is enough to break soft candies like Reese’s or Rolos into pieces. If using a hand mixer, coarsely chop the candy bars first, then fold them into the dough. Press the sticky dough evenly into the prepared pie crust, spraying your hands lightly with cooking spray to prevent sticking. Reserve some chopped candy to press onto the top of the pie for decoration. Use the tines of a fork to gently press and pattern any exposed edges of the crust. Bake the pie for 24 to 30 minutes, until the crust is browned and the top turns golden; the center may still jiggle slightly. Allow the pie to cool completely before slicing. Serve plain or with whipped cream or ice cream for an extra indulgence.