Add the marshmallows, water, and corn syrup to a large microwave-safe bowl. Use a bowl that holds at least 3 times the volume, because marshmallows balloon as they heat. Microwave for 30 seconds, stir, then continue in 20-second bursts. Stop when the mixture is puffed, smooth, and no solid pieces remain, usually 1 to 1½ minutes in total.

Add the vanilla and salt right after melting. Stir with a silicone spatula for about 30 seconds until the mixture looks like a thick, sticky glaze. Don't worry about lumps at this point. A few small unmelted flecks will dissolve during whipping.

Beat with a hand mixer or stand mixer on medium-high for 2 to 3 minutes. The cream is ready when it turns bright white, glossy, and visibly lighter, and it forms soft ribbons when you lift the beaters. If it still looks dense and sticky, beat another minute. Over-beating is rarely a problem here.

Warm marshmallow cream is pourable and spreads easily over brownies or warm cookies. For a thicker, spoonable texture, let it stand 15 to 20 minutes at room temperature. Transfer to a clean jar and cover tightly once cool.

Keep it in an airtight jar in the refrigerator for up to 2 weeks. It stiffens when cold, so microwave it for 8 to 10 seconds and stir to loosen before using. Rebecca's Tip: Lightly oil your bowl, spatula, and beaters with neutral oil or cooking spray before you start. Marshmallow is extremely sticky, and a thin film of fat makes cleanup and transfer much easier without changing the flavor.