Preheat the oven to 350°F and line an 8x8-inch pan with parchment paper. Set aside.
In a large mixing bowl, whisk together the melted butter and brown sugar until smooth. Add the egg, egg yolk, and vanilla extract, mixing until fully incorporated.
In a separate bowl, combine the flour, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture, stirring until no flour pockets remain. Be careful not to overmix.
Spread the batter evenly into the prepared pan. To make the cinnamon sugar topping, mix the cinnamon and sugar together, then drop large spoonfuls over the blondie batter. Use a butter knife to swirl it in, creating a marbled effect.
Bake for 30 to 40 minutes, or until the edges are golden brown and set, while the center remains slightly doughy. The blondies will continue to set as they cool.
While the blondies cool, prepare the cream cheese icing. In a small saucepan over medium-low heat, whisk together the cream cheese, heavy cream, powdered sugar, vanilla extract, and a pinch of salt. Stir constantly until smooth and fully combined, then remove from heat and let cool slightly.
Once the blondies have cooled, drizzle them with the cream cheese glaze. Slice into 9 squares and enjoy!