Start with a grease free mixing bowl and whisk. Wipe with lemon juice or vodka to ensure.
Measure all ingredients using a scale.
Whisk sugar and meringue powder together in the bowl of a stand mixer with the whisk attachment.
Add egg whites to sugar mixture in the bowl until incorporated.
Create a double broiler by placing mixing bowl over a pot of simmering water until the sugar mixture hits 135˚F. Do not allow the bottom of the bowl to touch the water, you just want to heat bowl with the steam.
Whip on high speed for 20 min in stand mixer or until room temp.
Change to paddle attachment, adjust to medium speed and slowly add shortening, then butter.
Beat until fully incorporated and light in color.
***It might seem like it's “breaking” at some point, where the butter may look curdled. No worries, keep beating and it will come back together. It won’t generally happen so long as the temperatures are respected.
This can be made in advance and refrigerated. To reconstitute: microwave container of buttercream for 20 seconds, just barely removing the chill from the outside of the container. Place buttercream back into a stand mixer and beat on high until light and fluffy. Again, at this stage there may appear to be a “break” in the emulsion, but allowing it to continue to beat and warm to room temp will remedy the problem.