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Preheat the oven to 350°F.
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In a bowl, combine all the ingredients for the cookie layer with a pastry blender or pulse in a food processor.
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Pat into a buttered 12x7-inch flat glass baking dish.
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Bake until golden.about 10 minutes.
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While the first layer is baking, prepare the filling.
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In a mixing bowl, beat the eggs and the vanilla until the eggs are pale yellow, about 2 minutes.
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In another bowl, combine the brown sugar, flour, baking powder, salt, walnuts, and coconut.
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Add to the egg mixture and blend.
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Cool the baked first layer for 5 minutes, then pour the walnut coconut mixture over the crust and bake for 25 minutes or until the top is golden and the center is done.
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Transfer to a wire rack and cool completely before frosting.
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To prepare the frosting while the layers cool, all the frosting ingredients in a small bowl, until smooth.
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Spread over the top of the cooled cookie.
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Allow to stand for 1 hour, then cut into squares.
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Store in layers separated by plastic wrap in an air tight container.