COAT lightly a 9" by 12" pan with nonstick spray and set aside.
MELT butter in a microwave safe bowl. Toss marshmallows on top of the melted butter. Place bowl back in the microwave and heat for 2 minutes. Stir marshmallows and butter to integrate and return to the microwave to heat for an additional minute. Carefully pull bowl out of the microwave, stir well and set aside to cool for about 3 minutes. You don't want it to be super hot, otherwise it will melt the chocolates when added.
MELT the caramel cubes with milk for 1 minute in another microwave safe bowl. Remove from microwave very carefully and stir in 1 teaspoon of sea salt. Set aside.
POUR in the cereal, peanuts, cranberries, pretzels, pumpkin seeds and chocolate candies into marshmallow mixture immediately. Stir with a greased spatula until evenly coated. Pour into the prepared pan and press evenly into pan with a greased piece of wax paper.
DRIZZLE melted caramel all over the top of the marshmallow trail mix treats using a spoon. Sprinkle the remaining teaspoon of coarse salt in top of caramel.
COOL completely at room temperature. Once cool, cut into 2" by 3" squares with knife. I like to heat my knife's blade over the stove for 5 seconds before making a cut. The treat cuts easier and you get neater edges.
STORE crispy treats in air-tight container at room temperature for up to five days.