Place 4 ounces of unsalted butter in a pan over low to medium low heat, keeping an eye on it and stirring occasionally.
The butter will go through a few stages before it starts to brown.
First, the white part (the milk solids) will start to separate, then it will sink to the bottom. Once it sinks to the bottom, you need to watch it closely. That's when it will start to brown. Use your nose to tell when it's ready. It will smell slightly nutty. The little milk solids at the bottom will look like tiny specks. Watch them closely because that is what will turn slightly brown.
Once the butter is lightly browned, add the brown sugar and salt.
Cook on medium low heat for about 2 minutes, stirring constantly.
Turn off the heat and add the cinnamon. Stir until incorporated.
Set aside to let cool to room temperature.
Once at room temperature (very important), whip on high with the paddle attachment until it is lighter in color and very fluffy. If it doesn't turn light and fluffy, let it cool longer.
Pipe onto the macarons. Since this filling is pretty sweet, you won't need too much.