Before you start, remember that it's not safe to eat raw cornstarch as it can create bloating or carry bacteria. For an edible version, use powdered sugar instead, keeping in mind that the texture will be way stickier.
In a medium-sized microwave-safe mixing bowl, add marshmallow. Place the bowl in the center of the microwave and microwave for 30 seconds until the marshmallow puffs up.
Watch out the mixture is very hot, so use gloves to remove the bowl from the microwave.
Add vegetable oil to the bowl and stir the mixture with a spoon. It will not dissolve well and stay glossy and oily, so start adding cornstarch, 1 tablespoon at a time. The slime is forming as you stir more cornstarch. Adjust the amount of corn starch, adding more to make the slime less sticky and firmer. Use your hands only when the mixture reaches a safe temperature.
Marshmallow slime hardens quickly as the water molecules evaporate while playing with it. You can soften the slime by bringing it back to the microwave-safe bowl and microwave for 10 seconds. Always stir with a spoon at first to avoid burning yourself. Then when it's safe to touch, knead and play and adjust the texture by adding a pinch more cornstarch to dry it out or drops of oil if too sticky.