Start by preheating your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving a slight overhang on the sides for easy removal later. In a large mixing bowl, whisk together the melted butter and packed brown sugar until smooth and glossy. Add in the egg and vanilla extract and whisk again until the mixture looks creamy and well blended. In a separate small bowl, combine the flour, baking powder, and salt, then gently stir the dry ingredients into the wet ingredients until just combined — don’t overmix. Fold in the chocolate chips until evenly distributed. In another bowl, beat the softened cream cheese, granulated sugar, and vanilla extract until smooth and lump-free. Pour about three-quarters of the blondie batter into your prepared pan and smooth the surface with a spatula. Drop spoonfuls of the cream cheese mixture over the top, followed by dollops of the remaining blondie batter. Using a knife or skewer, gently swirl the two mixtures together to create a marbled effect — don’t overdo it or you’ll lose those beautiful ribbons of cream cheese. Bake for 25 to 30 minutes, until the edges are lightly golden and the center looks just set. A toothpick inserted near the center should come out with a few moist crumbs. Let the blondies cool completely in the pan before lifting them out by the parchment and cutting them into squares. Enjoy them warm, chilled, or even straight from the fridge — they’re delicious any way you slice them! Store leftovers in an airtight container at room temperature for up to three days or refrigerate for up to a week. For longer storage, wrap individual bars tightly and freeze for up to two months.