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Preheat oven 180°C (approx 350°F).
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Butter and flour three 9 inch square baking pans.
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Tap out any excess flour and set aside.
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In a medium bowl, combine the flour, baking powder, and salt, and mix well.
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In a large bowl, beat the butter and sugar together until light and fluffy, then beat in the egg yolks, one at a time.
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Alternately add the dry ingredients and milk to the batter, beginning and ending with the dry ingredients.
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In another large now, beat the egg whites until stiff, glossy peaks form then gently fold them into the batter.
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Stir in the vanilla.
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Scrape the batter into the prepared pans and bake for 20 to 30 minutes, or until a cake tester inserted in the center of each layer comes out clean.
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Let cool completely in the pans.
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To make frosting.
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In a large, heavy saucepan, melt the butter over low heat.
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Add the brown sugar and milk and stirring; bring the mixture to a boil.
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Remove from the heat and let cool.
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Gradually stir in the caster sugar until well blended and very smooth.
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Run a knife around the edges of each cake layer and un-mould onto wire racks.
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Stack the layers on top of each other and spread the frosting over the top of each layer then all around the sides with a knife.