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Preheat the oven to 400°F.
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Line a 10 x 15 jelly roll pan with foil, and coat with non-stick cooking spray.
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Using a large saucepan over medium-high heat, melt the butter. Remove from heat.
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Stir in the sugar, vanilla, and 3 of the eggs until smooth.
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Add the baking soda in pinches.
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Mix in the flour and stir until smooth.
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Pour half of the batter into the pan and spread in a thin layer.
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Press the chocolate chips into the batter.
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Beat the remaining eggs and cocoa into the batter until smooth. Mix in the walnuts.
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Pour into the pan on top of the first layer and spread evenly.
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Bake on the lowest rack for 18 minutes, until the top is dry but the interior is not yet firm.
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Store for up to one week, wrapped in plastic.