To pop 6 cups of popcorn at a time in the microwave, pour ¼ cup popcorn kernels in a clean paper lunch sack (I use a 12 ⅜ X 6 ⅛ X 4" brown paper lunch bag). Fold over the top several times. Microwave on high until there is a 2-second pause between pops (all microwaves vary, but this will probably take around 2 minutes, give or take 30 seconds). Shake the bag so that any unpopped kernels roll to one corner. Tear a tiny hole in that corner and shake out kernels. Pour popped popcorn into a bowl. Repeat entire process for a total of 12 cups popcorn.
Place butter in a small, heavy-bottomed pot. Set pot over medium-low heat and stir constantly as butter melts and foams. After a few minutes, butter will turn golden and brown specks will form. When the foam subsides and the butter begins to smell nutty, remove from the heat. Butter should be deep golden brown but not black or burnt.
Remove pot from heat and stir in vanilla (be prepared for butter to foam up quite a bit). While brown butter slightly cools, mix sugar and cinnamon together in a small bowl until well combined.
Slowly drizzle brown butter over popcorn, allowing any black/dark brown sediment at the bottom of the pot to remain behind. Gently stir popcorn until evenly coated. Sprinkle half of the cinnamon sugar mixture over buttered popcorn; stir, and repeat with remaining cinnamon sugar. Serve immediately.
If you don't have salted butter, stir ⅛ to ¼ teaspoon (to taste) of salt into brown butter before pouring over popcorn.
If you don't want to mess with browning the butter, regular melted butter works just fine, too. :)