- 1 1/3 cups unbleached all purpose flour
- 1 teaspoon baking powder
- ½ teaspoon salt
- ½ cup (1 stick) unsalted butter at room temperature
- ¾ cup packed light brown sugar
- ½ cup granulated sugar
- 2 large eggs
- 2 tablespoons instant coffee powder dissolved in 2 tablespoons warm water
- 1 teaspoon vanilla extract
- 1 cup (5¾ ounces) plus 3 tablespoons white chocolate chips
- Position a rack in the middle of the oven. Preheat the oven to 350°F. Butter a 9-inch square baking pan and line the pan with a piece of parchment paper that is long enough to extend over two opposite sides of the pan. Butter the paper.
- In a small bowl, stir the flour, baking powder, and salt together. Set aside.
- In a large bowl, using an electric mixer on medium speed, beat the butter, brown sugar, and granulated sugar until smoothly blended, about 1 minute. Stop the mixer and scrape the sides of the bowl as needed during mixing. Add the eggs, dissolved coffee, and vanilla and mix until blended, about 1 minute. The mixture may look slightly curdled. On low speed, add the flour mixture, mixing just until it is incorporated. Mix in 1 cup of the white chocolate chips until evenly distributed.
- Scrape the batter into the prepared pan. Bake just until the top feels firm when lightly touched and a toothpick inserted in the center comes out clean, about 30 minutes.
- Transfer the pan to a wire rack. Immediately sprinkle the remaining 3 tablespoons white chocolate chips over the top. Let the chips sit for 10 minutes, then use the back of a teaspoon to gently smear the melted chips to create large marbleized swirls of white chocolate. The swirls will not completely cover the bars. Cool until the topping is firm, about 1 hour. These are thin bars and may sink slightly in the center as they cool, because the center is especially moist.
- Loosen the sides of the bars from the unlined sides of the pan and use the ends of the paper to lift the bars from the pan. Use a large sharp knife to cut the bars into 16 or 25 pieces and then a wide spatula to help slide the blondies off the paper.
- The blondies can be covered and stored at room temperature for up to 3 days.