The incredible punch of flavor from this buttercream comes from the super-strong cold brew incorporated into the meringue, and the brown sugar adds a delicious caramel undertone! Try it, it is entirely worth the effort.
Place your coarsely-ground coffee beans in a lidded container and add the 4 cups water, stirring to fully saturate the beans. Cover and keep in the fridge or just on the counter overnight, or at least 8 hours. Strain through a cheesecloth or a clean dish towel you don’t mind staining, and set the cold brew aside, discarding the used grounds.
Place your powdered egg whites (if using) in a small bowl and top off with the 14 tablespoons cold brew. Whisk gently but vigorously until you no longer see lumps. Pour into the bowl of your stand mixer and set aside.
Place the 1/2 cup cold brew into a large, deep saucepan and add the brown sugar. It is important that the saucepan be deep because brown sugar syrup tends to boil up higher than white sugar, and you don’t want to have to deal with a syrup spill, trust me! Set over medium heat, stirring occasionally. Use a pastry brush dipped in water to melt the sugar crystals that form on the sides of the pan. When the syrup begins to boil, reduce the heat to low and insert a candy thermometer into the syrup. Allow it to continue to boil while moving on to the next step.
Using the whisk attachment, whip the whites until frothy. Then, if using, sprinkle in the pinch of cream of tartar (this improves the strength of the meringue).
Keeping an eye on the temperature of the sugar syrup, increase the speed of the mixer to high to achieve stiff peaks. Ideally, you will time this to coincide with the sugar syrup reaching 240° F. If your whites reach stiff peaks before the sugar syrup reaches the correct temperature, reduce the mixer speed to the lowest setting and leave running until the syrup is ready.
When the syrup reaches 240° F, increase the mixer speed to the highest setting. Remove the saucepan from the heat and slowly pour the hot syrup down the side of the bowl into the egg whites, being very careful to avoid allowing the stream to hit the whisk and spatter. Pour slowly and steadily until all the syrup has been added, and continue beating the meringue until it becomes light and fluffy and holds its shape exactly when the beater is removed, roughly 3-5 minutes.
After this time, the meringue may still be hot—if so, place the bowl in the refrigerator (or freezer) for 10-15 minutes until it has cooled all the way through.
When the meringue has cooled, place the bowl back on the mixer, replacing the whisk with the paddle attachment. Begin mixing on low, and add the butter 1/4 cup (half a stick) at a time, beating well after each addition. Once the butter is added, increase the speed to medium and beat the buttercream until it becomes light and fluffy. It may look soupy or curdled shortly after all the butter is added—this is okay! Just keep on beating it and it will whip up into a light, fluffy cloud.
Add the vanilla additional grounds, if using, and beat on low speed until incorporated. Use straight away or keep in the refrigerator for up to 2 weeks in an airtight container, bringing to room temperature and beating until smooth to use.