Brown Sugar Swiss Meringue Buttercream
5 egg whites
1-1/4 packed cup brown sugar
1/4 teaspoon salt
3 sticks butter (24 tablespoons), cool room temperature then cut into small cubes
2 teaspoons vanilla extract
1 tablespoon cinnamon
Make a double boiler by placing the mixing bowl of a stand mixture over a sauce pan of boiling water.
Add the egg whites, brown sugar and salt. Whisk continuously until temperature reaches 140°F or until the sugar is completely dissolved and foamy.
Attach the bowl to the stand mixture fitted with a whisk attachment and begin whipping until the meringue is fluffy and the bottom of the bowl no longer feels warm (about 7 minutes; be patient; listen to music).
Add the butter, one at a time, while whisking. At this point, your frosting may break and curdle. You may want to start swelling up with tears and think you have to start over. It's not over. Dry your eyes, my grasshopper. Keep adding butter and whisking until the frosting comes back together to a light and silky texture.
Add the vanilla extract and cinnamon. Whisk until combined.