Preheat the oven to 350 degrees F and set the rack to the middle position. Place two overlapping sheets of greased parchment or foil in a 9- by 13-inch baking dish, so they form a cross and hang over the ends by at least 1 inch.
Whisk together the flour, baking powder and salt in a small bowl. In another bowl, mix the melted butter and brown sugar until combined. Add in the eggs and vanilla, and mix until combined. Add the dry ingredients to the butter mixture and gently fold with a spatula until uniform. Fold in the peanut butter chips and peanuts.
Pour the batter into the baking dish and spoon on the preserves. Using a skewer or knife, make swirly patterns into the batter with the preserves. Bake until the top is shiny and a cake tester comes out clean, 30 to 35 minutes. Cool in the dish on a wire rack. Using the ends of the foil or parchment, lift the blondies out the dish. Cut into 24 squares and serve!
Recipe courtesy of Jeff Mauro