Beat (whisk) the egg whites in high speed for around 1 minute till it get bubbles
Add half sugar from group A then keep mixing on high speed for another 1 minute
Then add remaining sugar and keep beating on high speed till the egg whites getting sharp and peak point. Then reduce and turn off machine, and leave it
Bring the pot of group B then heat till it reach 235 F (117 C), very carefully at this step.
Then turn back the mixing machine on high speed, slowly drizzle the hot sugar into the mixer immediately.
Whip until the mixture becomes cool, whiter and soft peak
Change to paddle attachment then unsalted butter to running on slow speed, one by one.