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Preheat oven to 350 degrees F. Grease and flour a 2 inch deep 9 inch baking pan; A spring form pan also works well.
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Sift together the flour and baking powder and set aside.Cream together the butter and sugar well. Beat in the eggs, one at a time, beating well after each addition. Beat in the vanilla and sour cream.
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Fold in the dry ingredients and divide the batter in half. To one half fold in the orange zest and to the other half fold in the cocoa and milk.
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Drop the batter by large spoonfuls randomly into the prepared pan, alternating between the chocolate and orange batters. Swirl slightly with the handle of a wooden spoon if you like for a more marbled effect.
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Bake for 40-45 minutes or until a wooden toothpick inserted in the center comes out clean.
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Let the cake rest in the pan for about 15 minutes before poking the entire surface of the cake with a wooden skewer. Pour or brush on the Grand Mariner liqueur.
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Remove cake, right side up to a wire rack to cool completely.When cool, top with truffle glaze.
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For the truffle glaze melt together the chocolate chips, butter and whipping cream in a double boiler.
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When melted you can whisk in a small splash of Grand Marnier if you like. Spread over cooled cake and top with toasted chopped hazelnuts.