Preheat oven to 350°F (175°C) and line an 8x8-inch pan with parchment paper, leaving a bit of overhang for easy lifting.
In a bowl, whisk melted butter and brown sugar until smooth, then mix in the egg and vanilla.
Stir in flour, baking powder, and salt until just combined, then fold in chocolate chips.
In a separate bowl, beat cream cheese, sugar, egg yolk, and vanilla until creamy.
Spread the blondie batter evenly in the pan, spoon the cream cheese mixture on top, and swirl gently with a knife for a marbled look.
Bake for 25–28 minutes, until the edges are golden and a toothpick comes out with a few moist crumbs.
Cool completely, then chill for 30 minutes before slicing for clean, neat bars.