1. Preheat oven to 350°. Line bottom and sides of a 9- x 5-inch loaf pan with heavy-duty aluminum foil, allowing 2 to 3 inches to extend over sides. Lightly grease foil with cooking spray.
2. Beat egg and sugar at medium speed with an electric mixer 2 minutes or until creamy; add peanut butter and butter, beating until blended.
3. Stir together flour and next 3 ingredients in a medium bowl; whisk in egg mixture and buttermilk just until blended. (Batter will be slightly lumpy.) Stir in mini-morsels.
4. Spoon about 2/3 cup Peanut Streusel Topping on bottom of prepared pan. Spoon half of batter over streusel, gently spreading batter to sides of pan to cover streusel. Repeat procedure once. Sprinkle remaining 2/3 cup Peanut Streusel Topping over batter, gently pressing topping into batter.
5. Bake at 350° for 1 hour and 5 minutes or until a long wooden pick inserted in center comes out clean, shielding with foil after 45 minutes, if necessary. Cool in pan on a wire rack 10 minutes. Lift loaf from pan, using foil sides as handles. Place loaf on a wire rack. Carefully pull down sides of foil. Cool loaf completely (about 1 hour).