PREP: Line a very large sheet pan (15x21 inches) with parchment paper or foil. Lightly spray with cooking spray.
CEREAL: On the prepared pan, combine the 3 cups Rice Chex cereal, 3 cups Corn Chex cereal, 3 cups Golden Grahams cereal, 1 ½ cups mixed roasted and salted nuts, and 2 cups sweetened coconut. Toss together with your hands.
SYRUP: Place the ¾ cup unsalted butter (cut into tablespoons), the 1 cup white sugar, and the 1 cup corn syrup in a pan and stir over medium-high heat until butter is melted. Bring the mixture to a boil. Once boiling, boil for 2 minutes, stirring constantly. After 2 minutes, remove from the heat and carefully add in the ½ teaspoon salt, 1 teaspoon vanilla (it may sputter), and optional ½ teaspoon coconut extract.
COAT CEREAL: Pour the prepared sugar mixture over the cereal mixture on the pan and, using 2 wooden spoons or a spatula and a spoon, toss to evenly coat. Make sure all the cereal/nuts/coconut are well-coated in the sugar mixture. Let mixture stand at room temperature until hardened and firm, about 1-2 hours. The mix won't ever fully firm up; it is meant to be sticky and gooey.
STORAGE: Store in an airtight container at room temperature for up to 5 days. The cereal and nuts do get softer/mushier each day the mix sits. We like this mix best the first or second day after it is made.
Video
Notes
Note 1: This mixture is sticky, gooey, and very sweet. If you'd like it less sweet and/or a bit firmer, add 1 extra cup of each of the cereals. (We like this mix just as written - sticky and sweet!)