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1
Position a rack in the middle of the oven.
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2
Preheat the oven to 350F.
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3
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.
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4
Butter the paper.
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5
In a small bowl, stir the flour, baking powder, and salt together.
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6
Set aside.
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7
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.
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8
Stop the mixer and scrape the sides of the bowl as needed during mixing.
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9
Add the eggs, dissolved coffee, and vanilla and mix until blended, about 1 minute.
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10
The mixture may look slightly curdled.
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11
On low speed, add the flour mixture, mixing just until it is incorporated.
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12
Mix in 1 cup of the white chocolate chips until evenly distributed.
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13
Scrape the batter into the prepared pan.
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14
Bake just until the top feels firm when lightly touched and a toothpick inserted in the center comes out clean, about 30 minutes.
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15
Transfer the pan to a wire rack.
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16
Immediately sprinkle the remaining 3 tablespoons white chocolate chips over the top.
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17
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.
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18
The swirls will not completely cover the bars.
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19
Cool until the topping is firm, about 1 hour.
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20
These are thin bars and may sink slightly in the center as they cool, because the center is especially moist.
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21
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.
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22
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.
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23
The blondies can be covered and stored at room temperature for up to 3 days.
