Lightly grease a 9x9 inch baking dish with cooking spray or oil.

In a large mixing bowl, combine gelatin and cold water, letting it bloom for about 10 minutes.

In a saucepan, combine sugar, corn syrup, and remaining water. Cook over medium heat until the mixture reaches 240°F (soft ball stage). Stir gently to dissolve the sugar completely.

Pour the hot syrup slowly into the bloomed gelatin while whisking continuously until thick and glossy, about 8-10 minutes.

Blend in vanilla extract and food coloring if desired.

Carefully pour the mixture into your prepared dish, smoothing it out with a spatula. Let it sit at room temperature for at least 4 hours or until fully set.

Dust a clean surface with powdered sugar, turn out the marshmallow slab, and cut into heart shapes using a cookie cutter or knife.

Roll each heart-shaped marshmallow in powdered sugar to prevent sticking, then store them in an airtight container for up to two weeks.