Prepare the pan. Preheat the oven to 350°F (177°C). Grease a 9×5-inch loaf pan with butter or nonstick cooking spray, or line it with parchment paper leaving an overhang for easy removal.
Combine butter and sugars. In a medium bowl, whisk together the melted butter, granulated sugar, and brown sugar together until combined. ½ cup (1 stick, 113 g) unsalted butter ½ cup (107 g) packed light brown sugar 6 Tablespoons (75 g) granulated sugar
Add egg and vanilla. Beat in the egg and vanilla until combined. Scrape down the sides and bottom of the bowl as needed. 1 large egg 1 teaspoon vanilla extract
Add dry ingredients. Add the flour, baking soda, and salt. Mix on low speed just until a thick blondie dough forms. Do not overmix. 1 cup + 2 Tablespoons (143 g) all-purpose flour ¼ teaspoon baking powder ¼ teaspoon salt
Add chocolate. Fold in the chocolate chips with a spatula. ¾ cup (130 g) semi-sweet chocolate chips
Fill the pan. Press the dough evenly into the prepared loaf pan. The mixture will be thick and sticky. Sprinkle a few additional white chocolate chips over the top.
Bake. Bake for about 20-25 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out with a few moist crumbs. I’d start checking around 18 minutes—you want these slightly underdone rather than dry.
Cool. Let the blondies cool completely in the pan before lifting them out and slicing.