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1. Preheat the oven to 375u00b0F (190u00b0C). Lightly spray or grease a 9-by-12-inch (23-by-30-cm) baking pan with cooking spray or butter.
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2. Combine the butter, the 2 cups chocolate chips, and the bittersweet chocolate in a metal bowl or the top pan of a double boiler and place over (not touching) simmering water. Heat, stirring, until all of the chocolate is melted and the mixture is smooth. Remove from the heat and let cool until still warm but not too hot to touch, about 5 minutes.
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3. In a large bowl, combine the eggs, sugar, and vanilla and stir until well blended. Pour the egg mixture slowly into the warm chocolate mixture, stirring until thoroughly combined. Let cool to room temperature, about 15 minutes.
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4. In a medium bowl, stir together the flour, baking powder, salt, the remaining 1 cup chocolate chips, and the crisped rice cereal, if using. Add to the cooled chocolate mixture and stir just until combined. Be careful not to overmix. Pour the batter into the prepared pan and smooth the top with a rubber spatula.
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5. Bake just until a toothpick inserted into the center comes out clean, about 35-40 minutes. Be careful not to overbake. Transfer to a wire rack and let cool completely in the pan, then refrigerate until well chilled, about 4 hours. Cut into bars and serve. Store leftover bars in an airtight container at room temperature for up to 3 days.
