Preheat oven to 350F. Line an 8-by-8-inch pan with aluminum foil, spray with cooking spray; set aside.
In a medium microwave-safe bowl, melt the butter, about 60 seconds. To the melted butter, add 1/4 cup peanut butter and heat for 30 to 45 seconds to soften the peanut butter. Stir until the melted butter-peanut butter mixture is smooth and combined. Add the brown sugar and stir to combine. Add the egg, vanilla, and stir to combine. Add the flour, salt, and stir until just combined, taking care not to over-mix or the blondies will be tough. Pour batter into prepared pan, smoothing it lightly with a spatula or offset knife if necessary.
To the top of the batter, add 1/4 to 1/3 cup peanut butter, dolloped out in 4 or 5 tablespoon-sized mounds. Add 1/4 to 1/3 cup jelly, dolloped out in 4 or 5 tablespoon-sized mounds. Using a knife, marble the peanut butter and jelly in a zig-zag pattern, up and back, back and forth, across the pan until peanut butter and jelly is dispersed and marbled; do not over-do it or surface will look muddy rather than swirled or marbled.
Bake for 22 to 27 minutes, or until edges begin to slightly pull away from sides of pan, center is set and golden, and a toothpick comes out clean; do not overbake. Allow blondies to cool for at least 45 minutes before slicing and serving. Store blondies in an airtight container at room temperature or in the refrigerator for up to 1 week, or in the freezer for up to 3 months.
The blondies are ripe for substitutions and variations. Add about 1 cup of any of the following stirred into the batter: peanut butter, chocolate, butterscotch, or white chocolate chips; toffee bits, fresh or frozen fruit including cherries, blackberries, raspberries, blueberries; diced or whole peanuts, slivered almonds; diced candy such as Peanut Butter Cups, Reese’s Pieces, Peanut M&Ms, Peanut Butter M&Ms or whatever strikes your fancy.
To keep gluten-free, use your favorite gluten-free flour blend.