Preheat the oven to 375 degrees.
Sift together the confectioner's sugar and the flour, and spin in a food processor for 30 seconds. Set aside.
In a large bowl, beat the egg whites and the cream of tartar with a hand mixer until they are foamy.
With the mixer on high speed, add granulated sugar a little at a time, until it is incorporated, then add the vanilla and salt.
Keep beating until stiff peaks form.
Sift the superfine sugar and flour onto the meringue, a little at a time, and gently fold in with a spatula until just mixed.
Gently spoon the batter into an ungreased angel food cake pan and bake 30-35 minutes until springy, or until a cake tester comes out clean.
Place pan upside down on top of a bottle or coffee cup. Let it hang and cool for 2 hours before using a knife to gently loosen it from the pan.
Combine the sugar and the lemon juice in a medium saucepan and heat over low heat until all the sugar is dissolved.
Add in the strawberries and stir to mix. Continue cooking over medium-low heat while the berries release their juices for about 10-15 minutes, until the mixture just comes to a slow boil.
Take the sauce off the heat and pour into mason jars, then store in the fridge to cool.
When you're ready to serve the cake, place the cake on a platter, and drizzle the strawberry sauce over it. Sprinkle the blueberries on top, and finish with a dollop of your favorite whipped topping.