Place the butter in a small sauce pan over medium-high and heat until completely melted. Once melted reduce the heat to low and add the rosemary sprigs. Continue cooking over low heat for 5-8 minutes, stirring occasionally. Remove the pan from the stove and let cool. Once cooled, strain out the rosemary and transfer the butter to a container. Place the butter in the refrigerator to let harden to a room temperature consistency. This can take up to an hour. We prefer to infuse the butter the day before.
Place the egg whites and sugar in a heat proof bowl. We prefer to use the metal bowl of our stand mixer.
Fill a pot smaller than your bowl with a couple inches of water and bring to a boil. Once boiling turn down to a simmer and set the bowl on the pot to create a double boiler. Continuously stir the egg whites and sugar until the sugar has fully dissolved or the temperature reaches 160 degrees on a candy thermometer.
Transfer the bowl to a stand mixer fitter with the whisk attachment and mix on high until the mixture holds stiff peaks, about 8-10 minutes. At this point the bowl should also be cool to the touch.
While the meringue is mixing cut the rosemary butter into 1 inch cubes.
When the meringue forms stiff peaks continue mixing on high and add the butter one cube at a time. Once all the butter has been added mix in the vanilla. After the vanilla has been added continue mixing on high for 5 minutes to create your silky smooth buttercream.