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Preheat oven to 300°F and grease an 8x8 baking pan.
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Combine unsweetened chocolate and butter in a medium saucepan and melt over medium low heat, stirring constantly until pieces are almost melted.
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Remove from the heat and stir until smooth.
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In a large bowl using an electric mixer on medium speed, beat eggs until light yellow in color--about 5 minutes.
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Add sugar and blend on low until thoroughly combined.
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Add vanilla and melted chocolate to the egg sugar mixture.
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Blend on low until smooth.
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Add the flour and mix thoroughly.
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Pour batter into greased pan.
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Smooth surface with a spatula, and sprinkle uniformly with chocolate chips.
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Bake on center rack of oven for 45-55 minutes.
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The batter should be set and a toothpick inserted into the center should come out clean.
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Do not overbake.
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Cool to room temperature.
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Cover and refrigerate for at least one hour.
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Cut and serve chilled.