Cereal Milk: In a jar or glass, add the 1 cup of milk and 1 cup rainbow marshmallow cereal. Mix together until the cereal is coated, then seal with a lid or plastic wrap. Refrigerate for at least 1 hour or up to overnight.
Cold Foam: In a jar or glass, add the ¾ cup heavy cream or half & half and ½ cup rainbow marshmallows. Mix together until the marshmallows are coated, then seal with a lid or plastic wrap. Refrigerate for at least 1 hour or up to overnight.
Assemble the Latte: In a small glass, place a spoonful of marshmallow creme in the bottom. Brew 1–2 shots of espresso (or coffee of choice) over the marshmallow creme. Set aside to cool slightly.
Strain the Mixtures: Strain the milk and cereal mixture so only the flavored cereal milk remains. Do the same for the heavy cream mixture.
Build the Drink: Fill a glass with ice. Pour about ¼–½ cup of the cereal milk over the ice. Next, pour the espresso over the milk.
Make the Cold Foam: In a small glass, add ¼ cup of the marshmallow-infused heavy cream along with the 1 teaspoon vanilla extract. Use a handheld frother to mix until the cold foam becomes thick and frothy. A standard frother works as well.
Finish the Latte: Gently pour the cold foam over the latte. Top with extra rainbow marshmallows. Insert a straw, stir slightly, and enjoy.