Preheat oven to 350oF. Spray 2 (6-inch) round deep cake pans with baking spray with flour.
For streusel: In a medium bowl, whisk together flour and brown sugar. Stir in peanut butter and butter until mixture is crumbly. Crumble with your fingertips until desired consistency is reached. Stir in peanuts.
For the cake: In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium-high speed until fluffy, 3 to 4 minutes, stopping to scrape the sides of the bowl. Reduce mixer speed to low. Add eggs, one at a time, beating well after each addition. Stir in vanilla.
In a medium bowl, whisk together flour, baking powder, and salt. Gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition. Divide three-fourths of batter between prepared pans. Spread with raspberry preserves, and top with remaining batter, smoothing tops with an offset spatula. Sprinkle with streusel.
Bake until a wooden toothpick inserted in center comes out clean, 40 to 45 minutes, loosely covering with foil to prevent excess browning, if necessary. Let cool in pan for 15 minutes. Run a sharp knife around edges of cake to loosen sides. Invert onto a plate, and then invert again onto a wire rack. Let cool completely. Garnish with warmed peanut butter, if desired.