Heat oven to 350°F. Spray 9-inch pie plate with cooking spray. In small bowl, mix flour, baking powder and salt; set aside.
In large bowl, beat brown sugar, softened butter and peanut butter with electric mixer on medium speed until light and fluffy. Beat in egg and vanilla until blended. Stir in flour mixture until just blended. Spread batter evenly in baking dish. Sprinkle candy on top; press lightly into batter.
Bake 16 to 19 minutes or until center just jiggles slightly and edges are light brown. Cool completely, about 3 hours or until chocolate is set.
In medium bowl, beat frosting ingredients with an electric mixer until light and fluffy; adding more milk if necessary. Pipe or spread frosting on edges of cooled pie. Decorate with sprinkles as desired.
Cut into wedges. Store covered in airtight container at room temperature.