Position the oven racks in the upper and lower thirds of the oven, then heat the oven to 350°F (175°C).
Place the gluten-free flour blend and cornstarch in a medium bowl. Whisk until evenly combined, then set the mixture aside.
Transfer the granulated sugar to a blender or food processor. Pulse briefly until the crystals become noticeably finer.
Do not process the sugar into a powder. The goal is a superfine texture that will dissolve more readily in the whipped egg whites.
Place the room-temperature egg whites, cream of tartar, and salt in the bowl of a stand mixer fitted with the whisk attachment. A large glass bowl and electric hand mixer may also be used.
Beat at medium speed, approximately level 6 out of 10, for about 1 minute, or until the whites appear very foamy.
Pour in the vanilla extract and continue mixing only until it is incorporated.
Reduce the mixer to medium-low speed, approximately level 4 out of 10. Gradually sprinkle the superfine sugar into the foamy egg whites, allowing it to dissolve as it is added.
After all the sugar has been incorporated, increase the mixer to medium-high speed, approximately level 8 out of 10.
Continue whipping until the meringue becomes glossy and forms medium peaks. When the whisk is lifted, the mixture should retain its shape while the peak bends slightly at the tip. The full whipping process should take approximately 3–4 minutes.
Add the reserved flour and cornstarch blend to the meringue in three separate portions. Use the mixer on low speed or fold with a spatula, combining each portion only until no dry streaks remain before adding the next.
After the final addition, gently stir the batter with a spatula to ensure the dry ingredients are evenly distributed without unnecessarily deflating the mixture.
Level the surface with the back of a spoon or an offset spatula.
Place the pan on the middle oven rack and bake for 45–47 minutes, or until the surface develops a deep golden-brown color.
Angel food cake is more likely to collapse when underbaked, so a slightly longer bake is preferable to removing it too early.
Immediately after removing the cake from the oven, turn the pan upside down. Balance it over a foil-covered bottle or use the pan’s built-in cooling feet, when available.
Allow the cake to cool completely in the inverted position to help preserve its height and prevent sagging.
Once fully cooled, return the pan to an upright position. Run a knife carefully around the edges to release the cake.
Cut the cake with a serrated knife. Serve each slice with whipped cream and fresh berries.