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Preheat oven to 375 F. Lightly coat a 10 cup Bundt or tube pan with oil and dust it with flour (We made ours in two loaf pans.)
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Partially fill the bottom of a double boiler with water and bring to a boil over high heat: reduce to a simmer. Put the chocolate and 1/4 c of oil in the top pf the double boiler. Heat just until the chocolate melts: remove from heat and stir until well combined.
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Combine the eggs and sugar in a large owl and beat with an electric mixer until fluffy. Slowly beat in the remaining 3/4 c oil, chocolate mixture, beets and vanilla.
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Sift together all purpose and whole wheat flours in a large bowl. Stir in baking soda and salt. Gently stir the flour mixture into the chocolate mixture just until flour is mixed in. Pour batter into prepared pan.
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Bake until a toothpick inserted near the centre comes out clean, about 45 minutes. Remove the pan from the oven and set it on a wire rack to cool for 30 minutes
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Carefully remove the cake from the pan and let cool on the rack. When completely cool, dust with powdered sugar.