This classic treat is a fine marriage of sweet and salty. With the warmth of dry mustard and the pop of yellow mustard seed, these squares are sure to please kids (and grown ups!) of all ages.
Grease a 9x13-inch baking dish with the canola oil and line it with parchment paper so the sides overhang.
In a large pot over medium-low heat, brown the butter. It will get foamy, then turn clear and golden then start to brown and smell incredibly nutty and wonderful. Stir frequently, scraping up brown bits from bottom of pot.
Turn heat to low and sprinkle in the dry mustard and sea salt.
Add the marshmallows, stirring constantly until the mixture is smooth and melted.
Remove from the heat and stir in the cereal and yellow mustard seed, stirring until evenly coated.
Scrape the mixture into a the prepared pan. You’ll need a large wooden spoon to help you. Work quickly, as the sticky stuff is easier to get out of pot while still hot. Have a bowl of cold water nearby and use wet hands to press the mixture into the pan. Smooth the top and let it rest for a couple of hours before cutting into squares.
Have a bowl of cold water nearby and use wet hands to press the mixture into the pan.