1. Preheat oven to 375°F. Grease a 9-by-13-inch baking pan and line with foil, letting ends of foil hang 3 inches over the 2 long edges. Grease foil.
2. Combine sugar and butter in a large pan over medium heat. Cook, stirring often, until butter has melted and sugar has dissolved (mixture will not look combined). Remove from heat and let cool slightly. Whisk in 2 whole eggs and vanilla.
3. In a bowl, whisk flour with baking powder and salt. Gradually add flour mixture to sugar mixture; mix until smooth. Fold in butterscotch chips. Spread evenly in pan.
4. Using an electric mixer on medium speed, beat cream cheese with egg yolk and Nutella until smooth. Drop spoonfuls of Nutella mixture over batter in pan and use the tip of a small knife to swirl it in, making a decorative pattern. Do not scrape bottom of pan.
5. Bake until blondies are just set in the center and a toothpick inserted into center comes out clean, 25 to 30 minutes. Let cool in pan on a wire rack, then cover and refrigerate until thoroughly chilled, at least 2 hours. Run a sharp knife along edges of pan to loosen, then remove blondies from pan using foil handles. Cut into squares and serve at room temperature.