Preheat the Oven: Preheat your oven to 350°F (175°C). Grease or line an 8×8-inch baking dish with parchment paper for easy removal.

1. Prepare the Blondie Batter: In a large bowl, combine the melted butter, peanut butter, granulated sugar, and brown sugar. Stir until smooth and creamy. Add the eggs and vanilla extract and whisk until well combined. In a separate bowl, mix together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
2. Add Jam to the Batter: Pour the blondie batter into the prepared baking dish, spreading it evenly. Drop spoonfuls of the fruit jam or jelly over the batter, then gently swirl it into the batter using a knife or toothpick to create a marbled effect.
3. Add Peanut Butter Swirl (Optional): Microwave the 1/4 cup of peanut butter for about 20 seconds until it’s slightly melted. Drizzle the peanut butter over the batter and use a knife to swirl it into the top.
4. Bake the Blondies: Bake for 25-30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. Be careful not to overbake, as the blondies will continue to cook in the pan once removed from the oven.
5. Cool and Serve: Allow the blondies to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely. Once cooled, cut into squares and enjoy!