Beat the butter and mayonnaise in the bowl of a stand mixer fitted with the paddle attachment, on high, until light and fluffy, 3 to 4 minutes. With the mixer on low, slowly add the melted chocolate. Continue mixing on low speed until all of the chocolate is incorporated, stopping to scrape down the sides of the bowl as needed.
Turn off the mixer and add one-third of the sugar. Mix on low to combine, stopping to scrape down the sides of the bowl. Repeat until all of the sugar has been incorporated. Add the salt and continue to beat until the frosting is smooth and lightens slightly in color, 2 to 3 minutes.
Use immediately or store in an airtight container at room temperature for up to 4 hours or refrigerate for up to a week. Bring to room temperature before using chilled frosting.