In a stand mixer, beat the room temperature butter until it's light and fluffy, about 1 to 2 minutes. Scrape down the bowl, then slowly add the powdered sugar and salt. Scrape again, then mix in the heavy cream and vanilla until just combined.
Next, get ready to toast the marshmallows. Set your broiler to 500 degrees F (or your broiler's highest setting) and let it warm up for about 1 minute. Line a rimmed cookie sheet with parchment paper and arrange the marshmallows in a single layer. Broil for about 5 minutes, watching them closely, until they're golden brown and melty. Keep an eye on them, as broiler times can vary depending on your oven rack placement.
Immediately scrape the toasted marshmallows into the mixer with the butter mixture and beat until the frosting is fluffy. Set this aside; it won't be quite thick enough to frost the cupcakes yet, but it will firm up while the cupcakes bake and cool. Finally, line a standard muffin tin with cupcake liners and preheat your oven to 350 degrees F.