Preheat oven to 250°F. Line 2 rimmed baking sheets or sheet pans with parchment paper.
In a saucepan over medium heat, combine the butter and brown sugar. Bring to a rolling boil, stirring occasionally. The mixture will become very hot and bubbly. Remove from heat and stir in the vanilla extract.
In a large bowl, combine the oats, salt and any ingredients you've selected from the Nuts and Crunch, Seeds, and Flavor Boosters categories.
Pour the hot syrup mixture over the oat mixture. Stir until everything is evenly coated.
Transfer the granola to the prepared baking sheets and spread into thin, even layer.
Bake for 45 minutes, without stirring for maximum crunchiness.
Remove from the oven and allow the granola to cool completely on the pan without stirring. The clusters will continue to form as the granola cools.
Once completely cool, stir in any Sweet Mix-Ins and break the granola into clusters.