In a large pot over very low heat, melt the butter and continue cooking gently until it turns golden brown and smells nutty, about 5–7 minutes. Stir frequently and don't walk away — brown butter goes from perfect to burnt quickly.

Add the marshmallows and stir gently on low until completely melted and smooth.

Take the pan off the heat. Stir in the orange blossom water, orange zest, and orange essential oil until combined.

Add the Rice Krispie cereal and stir until everything is evenly coated.

Pour into a greased or parchment-lined baking dish and gently press into an even layer. (Don't pack too firmly — that's what makes them dense.)

Let sit until cooled and firm, about an hour if you can stand to wait that long. 

Zest a bit more orange over top, cut into squares and enjoy.