Prepare the Marshmallow Base:

1. Bloom the Gelatin: In a small bowl, combine gelatin with ¼ cup cool water. If using a vanilla bean, scrape seeds into the water.

2. Make the Sugar Syrup: In a stainless steel pot, mix ¾ cup water, corn syrup, sugar, and salt. Heat over medium, stirring until sugar dissolves.

3. Cook the Syrup: Attach a candy thermometer to the pot. Increase heat to medium-high and cook the syrup to 250°F (around 8 minutes).

4. Combine Syrup and Gelatin: Pour the syrup into a stand mixer bowl, add bloomed gelatin, and beat on low speed until gelatin dissolves.

5. Whisk the Mixture: Increase the mixer speed to medium-high and whisk for 10 minutes until thickened and tripled in volume. Transfer the mixture to a container, cover, and rest at room temperature for at least 2 hours.

Make the Buttercream:

1. Beat the Marshmallow Base: Place the marshmallow base into the stand mixer and beat on medium speed to loosen it.

2. Add the Butter: Gradually add softened butter, 1 tablespoon at a time, mixing each addition fully before adding more. Scrape down the sides of the bowl as necessary.

3. Whip Until Creamy: Continue mixing until the buttercream is smooth, light, and fluffy. If too firm, warm it slightly to soften; if too soft, refrigerate briefly and rewhip.

Customize or Use as Desired:

• Use immediately for frosting cakes or cupcakes.

• For storage, refrigerate for up to 3 weeks. Allow it to come to room temperature before use, and rewhip if necessary.