Special equipment: a 10-inch angel food cake tube pan
Preheat the oven to 350 degrees F. Sift the cake flour with 3/4 cup superfine sugar; set aside.
Put the egg whites, 2 teaspoons lemon juice, the cream of tartar, almond extract and salt in the bowl of a stand mixer fitted with a whisk attachment. Whip on medium speed until frothy. Increase the speed to high, slowly add the remaining 3/4 cup sugar and beat until stiff peaks form.
Remove the bowl from the mixer and sift the cake flour mixture over the whites. Add the poppy seeds and use a rubber spatula to fold in the flour and poppy seeds until completely combined. Pour the batter into an ungreased 10-inch angel food cake tube pan and bake until golden brown, about 30 minutes. Invert the pan and cool for 1 hour, then remove from the pan and turn upright onto a serving plate.
Whisk the lemon zest and the remaining 3 tablespoons lemon juice with the confectioners' sugar and just enough water to make a smooth, pourable glaze. Pour the glaze over the cooled cake. Let the glaze set for 1 hour before serving.