Add the heavy cream and pasteurized egg whites to the bowl of a stand mixer fitted with the whisk attachment. Mix for 30 seconds.
Add the meringue powder, salt, and powdered sugar. Mix on low until the sugar is moistened, then whip on medium-high speed for 5 minutes.
Cut the softened butter into chunks. Add it gradually while mixing, allowing each addition to begin incorporating before adding more.
Add the vanilla extract and butter flavoring. Whip for another 2 minutes.
Replace the whisk with the paddle attachment. Beat on high speed for 5 minutes, scraping the bowl as needed, until the buttercream is smooth and silky.