Pour the corn syrup into one shallow dish and place the green sugar in a separate shallow dish.
Dip the rim of the serving glass evenly into the corn syrup, then immediately press it into the green sugar until the edge is thoroughly coated. The corn syrup helps the sugar adhere securely and produces a vivid Halloween-themed rim.
Fill the decorated glass with ice cubes and set it aside. Keeping the glass chilled helps the finished cocktail remain cold and reduces warming from the glassware.
Gather the remaining drink ingredients and prepare a cocktail shaker while the glass chills.
Fill a cocktail shaker with ice cubes. Pour in 2 fl oz vodka, 2 fl oz blue curaçao, and 2 fl oz grenadine.
Secure the shaker firmly and shake vigorously for 10–15 seconds, or until the liquid is thoroughly chilled and evenly combined. Tito’s vodka may be used for a particularly smooth base that allows the sweeter flavors to remain prominent.
Strain the chilled cocktail mixture over the ice in the prepared glass.
Slowly pour chilled cola over the drink to top it off. A gentle pour helps retain the visual layering and allows the grenadine to form its characteristic red swirls through the blue-colored base. The cola supplies additional sweetness and carbonation while enhancing the cauldron-like Halloween appearance.