Preheat the oven to 350 degrees . Grease a 9-inch cake pan and line the bottom with parchment paper. In a small bowl, combine the flour, baking powder and 1/4 teaspoon salt. In another small bowl, combine the milk and 2 egg whites.
Using an electric mixer, beat the butter and 3/4 cup sugar on high speed until fluffy, about 5 minutes; beat in the lemon peel. On low speed, beat in the flour and milk mixtures alternately, a little at a time, until the batter is smooth; pour into the prepared pan and bake until a toothpick inserted in the center comes out clean, about 30 minutes. Let cool on a wire rack for 5 minutes, then invert onto the rack to cool completely.
In a small saucepan, bring 1/4 cup water, the lemon juice, remaining 1 cup sugar, the cream of tartar and a pinch of salt to a boil, stirring, over medium-high heat until the sugar dissolves. Add the remaining 2 egg whites to a medium bowl, then slowly add the hot sugar mixture and, using the mixer, beat on medium speed until just combined and still warm, about 5 minutes. Increase the speed to high and beat until stiff peaks form, about 10 minutes. Set the frosting aside.
Place the angel food cake, wide end up, on a platter. Fill the center hole with frosting. Cut a 50-inch-long piece of foil wrapping paper or heavy-duty aluminum foil to the height of the cake, then fold like a fan, making 1/2-inch pleats. Wrap the foil around the cake and tape the seam. Coat the surface of the angel food cake with a thin layer of frosting. Place the single-layer cake on top and cover with the remaining frosting, piling it high. Top the "cupcake" with the candy "sprinkles" and a large candle.