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.