Step 1

Preheat the oven to 325 degrees F. Whisk the sugar, cream of tartar and salt together in a medium bowl.

Step 2

Add the egg whites and 2 tablespoons water to the bowl of a stand mixer fitted with the whip attachment. Beat on low speed until the whites are foamy, 1 to 2 minutes. Increase the speed to medium and slowly sprinkle in the sugar mixture (this should take about 3 minutes). Watching closely, continue beating until medium-soft peaks form, 5 to 7 minutes longer. (You want the mixture to droop when lifted out of the bowl. If the peaks stand straight up, you have taken it too far and the cake will sink during baking). Add the vanilla and mix until just combined.

Step 3

Remove the mixing bowl from the stand and working in three additions, very gently fold the flour into the egg whites until just incorporated. Spoon the batter into an ungreased 10-inch footed angel food cake pan and smooth the top.

Step 4

Bake until the top is golden and springs back when touched and a cake tester inserted into the middle comes out clean, 40 to 45 minutes. Remove from the oven and place the cake upside-down on a wire rack to cool completely in the pan, about 3 hours.

Step 5

Run a knife or a small offset spatula around the edges of the pan and gently tap it on the counter until it releases from the pan. Use a sharp serrated knife to slice the cake -- a regular knife will smash it. Serve with berries and whipped cream.