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.
Beat egg and sugar at medium speed with an electric mixer 2 minutes or until creamy; add peanut butter and butter, beating until blended.
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.)
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.
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).
Peanut Streusel Topping: Stir together 1 cup chopped dry-roasted, salted peanuts; 1/2 cup all-purpose flour; 1/4 cup butter, melted; 3 Tbsp. firmly packed light brown sugar; 2 Tbsp. granulated sugar; and 1/8 tsp. salt in a medium bowl.
Let stand 15 minutes or until mixture is firm. Crumble into small pieces. Makes about 2 cups; Prep: 10 min., Stand: 15 min.
Chocolate Chip-Peanut Streusel Bread: Prepare recipe as directed through Step 3; stir 3/4 cup semisweet chocolate mini-morsels into batter. Proceed with recipe as directed.
