Line a 13x9 pan with parchment paper.
Slice butter into smaller pieces and place in a large saucepan over medium-low heat
Keep stirring the butter until it is browned. It will have a nutty smell and be noticeably golden in color. See notes above for more specific instructions.
Once the butter is browned, add mini marshmallows. Stir quickly until marshmallows are melted and form a smooth mixture with the browned butter.
Remove the saucepan from the heat
Quickly stir in Rice Krispie cereal until it forms a consistent mixture.
Add in any mix-ins while the mixture is still warm.
Spoon the Rice Krispie treat mixture into your prepared pan. Press the Rice Krispie treats into the pan firmly, so they take the shape of the pan. It helps if you press the top flat with another piece of parchment paper.
Optional: Melt some melting chocolate (or regular chocolate with a bit of coconut oil) and drizzle on top.
Make sure everything has cooled before slicing your brown butter rice krispie treats into squares.