1 cup (8 ounces) unsalted butter, at room temperature
2½ cups powdered sugar
1 teaspoon vanilla extract
4 ounces semisweet or bittersweet chocolate, melted and cooled
1. Using the wire whisk attachment of your stand mixer, whip the butter on medium-high speed for 5 minutes, stopping to scrape the bowl once or twice.
2. Reduce the speed to low and gradually add the powdered sugar. Once all of the powdered sugar is incorporated, increase the speed to medium-high and add the vanilla, mixing until incorporated. Add the melted chocolate and whip at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl as needed to incorporate all of the chocolate.
You can store any unused buttercream in the refrigerator in an airtight container; let it come to room temperature and then give it a quick whip in the mixer before using.