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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
Put 1 cup of the chocolate chips in a heatproof bowl or the top of a double boiler and place it over a saucepan of barely simmering water (or the bottom of the double boiler); the water should not touch the bowl.
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6
Stir until the chocolate chips are melted and smooth.
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7
Set aside.
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8
Put the butterscotch chips in another heatproof bowl or the (clean) top of the double boiler and place it over the saucepan of barely simmering water (or the bottom of the double boiler); the water should not touch the bowl.
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9
Stir until the butterscotch chips are melted and smooth.
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10
Set aside.
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11
Sift the flour, baking powder, and salt into a medium bowl and set aside.
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12
In a large bowl, using an electric mixer on medium speed, beat the butter and brown sugar until smoothly blended, about 1 minute.
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13
Stop the mixer and scrape the sides of the bowl as needed during mixing.
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14
Add the eggs and vanilla and mix until blended, about 1 minute.
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15
The mixture may look slightly curdled.
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16
On low speed, add the flour mixture, mixing just until it is incorporated.
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17
Spoon 2 cups of the batter into a medium bowl.
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18
Use a large spoon to stir in the melted butterscotch chips until smoothly blended.
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19
Stir in the walnuts.
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20
Scrape the butterscotch batter into the prepared pan.
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21
Sprinkle the remaining 1 1/2 cups chocolate chips over the butterscotch batter.
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22
Stir the melted chocolate chips into the remaining batter in the bowl until smoothly blended.
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23
Drop spoonfuls of the chocolate batter over the chocolate chips and then use a metal spatula to spread the chocolate batter evenly over the chocolate chips.
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24
Some chocolate chips may get mixed into the chocolate batter-this is fine.
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25
Bake just until the top feels firm when lightly touched and a toothpick inserted in the center comes out clean or with a few moist crumbs clinging to it, about 40 minutes.
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26
If the toothpick penetrates a chocolate chip, test another spot.
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27
Transfer the pan to a wire rack to cool completely, about 1 hour.
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28
Loosen the sides of the brownie from the unlined sides of the pan and use the ends of the paper to lift the brownies from the pan.
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29
Use a large sharp knife to cut the brownies into 16 or 25 pieces and then a wide spatula to help slide the brownies off the paper.
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30
The brownies can be covered and stored at room temperature for up to 3 days.
