Using a small food processor, process the freeze dried strawberries into a fine powder. Gently remove the top of the food processor to avoid having the strawberry powder go everywhere.
Place the sugar and egg whites in a glass or metal mixing bowl (either one that is stand alone or part of a stand mixer), and set over a pot that is filled with 1"-2" of simmering water.
Whisk continuously until the mixture reaches 150ºF and/or the sugar has completely dissolved. You can check this by taking a small amount of the egg white mixture, rubbing it between two fingers. It should be smooth and not gritty in texture.
Carefully remove the mixing bowl from the simmering pot, wipe the bottom off with a towel, and beat the egg white mixture using a hand mixer or your stand mixer fitted with the whisk attachment on medium-high to high speed until it forms the stiff meringue peaks and (this is important) the bottom of the mixing bowl is cool to the touch. This can take anywhere from 10-15 minutes.
Add in the butter one piece at a time, allowing each piece to fully absorb into the meringue mixture. If it looks mushy or curdled, continue to beat on high speed. After you've added in all of the butter, switch out the whisk attachment for the paddle attachment and mix anywhere from 5-10 minutes. The meringue buttercream is ready when it is nice and smooth in texture.
Using a spatula, fold in the freeze dried strawberry powder.
When it's time to frost the cake, carefully cut the cake into two layers using a serrated knife and (optional) a rotating cake stand.
Chill the frosted cake for 10-15 minutes in the refrigerator (optional step) before running the icing comb over top. Add the sprinkles, then enjoy!
If not using the buttercream right away to frost the cake, you can cover the mixing bowl tightly with plastic wrap and place it in the refrigerator. Before using, bring the buttercream to room temperature (about 30 minutes), then whisk it again with your mixer to bring it back to the smooth consistency.