In a small saucepan, bring a small amount of water to a boil. Reduce the heat to a simmer.
In a medium heat-proof bowl, whisk together egg whites, sugar, and salt and place it over the saucepan with simmering water. (This is a makeshift double boiler. Make sure the bottom of the bowl doesn’t touch the water.)
Cook the egg white mixture until the sugar is completely melted and the mixture reaches 160°F (70°C), stirring continuously, about 5 minutes. (This is my favorite thermometer!). Now, remove the bowl with the egg white mixture from the heat. Then whisk it on medium speed for a minute, then slowly increase the speed to medium high and beat until stiff peaks form and the mixture is cool to touch, about 10 minutes. (It may take longer with a hand-mixer.)
Turn the mixer on medium speed and start adding butter, one tablespoon at a time. Make sure the butter is fully incorporated before adding the next piece. (NOTE: It’s normal if the buttercream starts to curdle halfway. Keep adding the butter!)
Keep whisking the buttercream until smooth, light and fluffy, about 10 minutes.
Meanwhile, place the freeze-dried strawberries in a plastic bag and crush them with a rolling pin into a fine powder. (Alternatively, you may use a small food processor.)
Now, switch to a paddle attachment and sift in the crushed freeze-dried strawberries. Beat the buttercream until well combined, about a minute.