Grease an 8x8 pan with melted butter, and set aside.
In a large pot, melt the butter over medium-low heat. Bring to a gentle simmer, and let cook until it gets foamy, browned, and toasty, about 4-5 minutes. Stir frequently to prevent burning!
Add the marshmallows, and stir until completely melted. Mix in the vanilla extract.
Stir in the puffy rice cereal and pretzels. Then, quickly fold in the chocolate chunks. Don't let the chocolate completely melt, or the flavor will overpower the brown butter! Some streaks are ok!
Evenly press the rice krispie mixture into the 8x8 pan. Put some extra pretzels or flaky salt on top, if desired! Refrigerate for 30-60 minutes.
Once cold, run a knife around the edge of the rice krispie treats, and remove the entire block of treats from the pan (it shouldn't fall apart). Cut into 9-16 squares. Store in an airtight container for up to two days! I personally think these are best day-of.