In the bowl of a stand mixer fitted with a whisk attachment, combine pasteurized liquid egg whites and powdered sugar. Mix on low speed until combined. Scrape the sides and bottom of the bowl to ensure the ingredients are well combined.
Turn the speed to medium-high and whip the egg white mixture until thick and opaque, approximately 3 minutes. (Note - the egg white mixture will not whip up to stiff peaks)
Add in salt and vanilla extract and continue to mix until combined.
Turn the mixer to low speed. Add room temperature butter, one tablespoon at a time, until all the butter is added. Allow the butter to mix for a few seconds in between each addition of butter.
Once the butter is added, turn the mixer to medium-high speed and whip the buttercream for 5-10 minutes. The buttercream may appear to curdle or separate - this is normal - just continue to whip the frosting. Scrape the sides and bottom of the bowl occasionally while whipping.
Once the buttercream appears smooth and creamy, add in the strawberry reduction and mix on medium-high speed for several minutes. The buttercream will appear to break, just keep whipping. Scrape the sides and bottom of the bowl to ensure everything is well combined.
Once the buttercream is smooth and silky it is ready to use, if desired, you can switch to the paddle attachment and mix on the lowest speed for several minutes to remove air bubbles and make the buttercream very smooth.