In a small saucepan, stir the sugar, water, and corn syrup together, cover with a lid, then heat to a boil.
After several minutes, remove the lid and either check for 242° F (117° C) on a candy thermometer, drip a drop into a bowl of cool water to see if it holds a ball shape, or check for a 6" to 8” thread.
About halfway through boiling, whip the room temperature egg whites just until stiff peaks start forming. Turn the mixer off and set aside.
When the sugar liquid is ready, resume the mixer to about medium-low speed then slowly and lightly stream the hot liquid into the egg whites very carefully.
Once combined, add the vanilla and cocoa powder then increase the mixer speed to whip the frosting into big billowy peaks.
Spread over a two layer 8”, two layer 9”, or a 9x13 cake. Store under a cake cover to prolong the time before frosting deflation.