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In a bowl set over simmering water, add the butter and 2 cups of the chocolate chips. Stir until the butter and chocolate melts. Remove from the heat and set aside to cool completely.
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Preheat the oven to 350 degrees F. Grease and flour a 9-inch tart pan with removable sides.
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In a large bowl, beat the eggs, sugar, coffee, and vanilla using an electric mixer on medium-high speed about 3 minutes, until lighter in color and well blended. Add the cooled chocolate and beat to combine.
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In a medium bowl, add the flour, baking powder, salt, 1 cup of the chocolate chips, and nuts. Stir to combine ingredients. Add the flour mixture to the chocolate mixture and stir until just combined. Pour the batter into the tart pan.
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Bake for 35 to 40 minutes, until the center is puffed. The top will crack. Cool the tart completely on a wire rack. Transfer the tart from the pan to a serving plate. Slice and serve with chocolate sauce, ice cream, and/or whipped cream.