Preheat & Prep: First things first, let’s get that oven ready! Set it to 325°F (that's about 160°C). Now, grease your 8-inch square baking pan with a little butter. Then, line it with parchment paper, letting the paper hang over the sides a little – this makes it easy to lift the blondies out later. Grease the parchment a little too!
Mix the Dry Stuff: In a medium bowl, whisk together 1 ½ cups of the flour, the baking powder, and the salt. This helps make sure everything is mixed evenly.
Make the Blondie Batter: In a bigger bowl, use your mixer to cream together the 10 tablespoons of softened butter and the brown sugar. Beat them until they look light and fluffy – this should take about 3 minutes. Then add in the eggs one at a time. Don't forget to mix well after each egg! Then, add the 1 ½ teaspoons of vanilla extract. Finally, gently stir in the dry ingredients until everything just comes together, don't overmix.
Make that Cream Cheese Swirl: In a separate bowl (make sure it’s nice and clean), beat together the softened cream cheese, granulated sugar, 2 tbsp of butter, the remaining ¼ cup of flour, 1 egg, and ½ teaspoon of vanilla. Mix until it’s super smooth and creamy.
Assemble the Blondies: Spread half of the blondie batter into your prepared pan. Next, plop about two-thirds of the cream cheese mixture over the blondie batter and spread it. Now, spoon the remaining blondie batter on top and smooth it out a bit. Use small dollops to finish with the remaining cream cheese mixture.
Create a Beautiful Swirl: Now for the fun part! Take a butter knife and swirl the cream cheese mixture through the blondie batter. I like to run the knife across, up, and down so that it gets nice and swirly!
Bake to Perfection: Pop your pan into the preheated oven and bake for 40–45 minutes. The top should be golden brown, and if you stick a toothpick in the middle, it should come out with just a few crumbs, not wet batter.
Cool and Enjoy: Let the blondies cool in the pan for about 15 minutes, then lift them out using the parchment paper overhang. Put them on a wire rack and let them cool completely before you cut them into squares. Trust me, this is worth the wait!