To make the filling, beat the egg whites, cream of tartar, and salt in a large bowl until the mixture is soft and fluffy., Add the confectioners' sugar, and beat at high speed until the mixture forms fairly stiff peaks. When you scoop some up in a spoon, it should hold its shape easily., Stir in the coconut flavor or vanilla., Toss the coconut with the flour, and add to the egg white mixture, stirring until thoroughly combined. Set it aside while you make the cake., Preheat the oven to 350°F. Lightly grease a a 9" to 10", 9- to 10-cup capacity bundt-style pan, or a 9" or 10" tube pan., Beat together the butter, sugar, and salt, first until combined, then until fluffy., Add the eggs one at a time, beating well after each addition, and scraping the sides and bottom of the bowl after you've added the first 2 eggs., Add the baking powder, then add the flour alternately with the milk, starting and ending with the flour. Mix until smooth. Stir in the grated lemon rind or lemon oil., Spoon about 1/3 of the batter into the prepared pan, smoothing the top with a spatula. You want to add enough to cover the bottom, and start to come up the sides; but not so much that you don't have enough left over to cover the filling., Distribute the stiff filling atop the batter, centering it within the ring of batter so it doesn't touch the sides of the pan. Pat it down gently., Dollop the remaining batter on top, again smoothing it with a spatula., Bake the cake for 50 to 55 minutes, or until a cake tester or toothpick inserted into the center comes out clean., While the cake is baking, make the glaze by stirring together the lemon juice and sugar. Set it aside., Remove the cake from the oven, and set it on a rack. After 5 minutes, run a knife around the edge of the pan to loosen, and turn the cake out onto a rack., Poke the hot cake all over with a cake tester or toothpick. Stir the glaze to combine, and immediately brush it on the hot cake. Let it sink in, then brush on more glaze, continuing until all the glaze is used up., Allow the cake to cool before slicing.