3 cups all-purpose flour, plus more for dusting pan
1 teaspoon baking powder
1½ teaspoons salt
1 cup (2 sticks) unsalted butter, at room temperature
1½ cups granulated sugar
2½ cups smooth peanut butter
2 large eggs
1 teaspoon vanilla extract
1½ cups jam or jelly, your favorite flavor
2/3 cup salted peanuts, roughly chopped
1. Heat oven to 350 degrees F. Grease a 9x13-inch pan with butter, and line the bottom with parchment paper. Grease the parchment, and coat inside of pan with flour; set aside.
2. Whisk together the flour, baking powder and salt; set aside.
3. Place the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed until fluffy, about 2 minutes. On medium speed, add peanut butter and eggs; beat until combined, about 2 minutes. Add the vanilla and combine. On low speed, add the flour mixture and mix until just combined.
4. Transfer two-thirds of the mixture to prepared pan; press evenly into pan. Using an offset spatula, spread the jam on top of the peanut butter mixture. Dollop remaining third of peanut butter mixture on top of jam. Sprinkle with peanuts.
5. Bake until golden, about 45 minutes. Transfer to a wire rack to cool; cut into about 36 1½x2-inch pieces.