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Preheat the oven to 350 degrees, and generously grease a 9×13 inch pan, optionally lining the pan with parchment paper. Set aside.
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In a large bowl, whisk together the melted butter, brown sugar, and regular sugar until smooth. Whisk in the eggs, milk, and vanilla extract until the mixture is smooth.
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Add the flour and salt to the mixture, and stir until combined. The batter will firm up as it cools, so moving quickly you'll want to spread the batter evenly into your prepared baking sheet.
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Dollop your peanut butter and jam all over the top of the blondie batter. Using a knife or an offset spatula, gently swirl the peanut butter and jam into the blondie batter.
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Sprinkle the flaky salt over the top of the batter evenly. Bake for 28-30 minutes, or until a toothpick inserting about 2 inches from the edge of the pan comes out clean. Cool completely, slice, and serve!