Preheat the oven to 325 degrees F. Lightly butter an 8-by-8-inch baking dish and put it in a roasting pan.
Whip the egg whites in a medium bowl using an electric mixer, on medium-high speed, until they form soft peaks. Then slowly pour in 1/4 cup sugar and continue to beat until the whites hold stiff, glossy peaks.
Beat the butter with the remaining 1/2 cup sugar in a large bowl, on medium speed, until creamy. Beat in the flour until combined and then add the coconut milk, lemonade concentrate, egg yolks and salt and beat until smooth. Fold a quarter of the egg whites into the coconut mixture to lighten the batter and then fold in the remaining whites.
Pour the batter into the prepared baking dish and add enough boiling water to the roasting pan to come halfway up the side of the dish. Bake until the top of the cake is slightly puffed and the middle is set but still jiggly, about 40 minutes. Remove from the water bath and cool on a wire rack for 10 minutes. Serve warm and garnish with toasted coconut if using.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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