Beat The Butter. Add softened butter to the bowl of a stand mixer (or a large mixing bowl). Using the paddle attachment (or a hand mixer), beat the butter on medium-high speed for 2-3 minutes, or until very smooth and light in texture. (It should look significantly lighter in color & have increased in volume.)
Add Most Of Your Ingredients. Scrape down the sides of the bowl and add black cocoa powder, powdered sugar, vanilla, salt, and 3 Tablespoons of heavy cream to the mixer.
Mix on LOW first. On LOW speed, pulse the mixer a few times so the powdered sugar and cocoa doesn’t poof out the sides of the bowl. (You can also add the dry ingredients in small amounts if you prefer to avoid messes)
Increase The Speed. Once it’s just mixed together, add 1-2 tablespoons more of cream, along with the corn syrup (if using). Increase the speed of the mixer to medium and mix for another 1-2 minutes, or until the buttercream is light and smooth. Ideally, you want it smooth and light without a lot of extra air bubbles.
Finish The Frosting. Use a flexible spatula to scrape down the sides of the bowl and press out any extra air bubbles, if you can. At this point, the frosting should be smooth and easy to spread or pipe. It should hold its shape on a spatula if you rotate it around (which means it will hold its shape when piped.)
Frost The Cupcakes. Transfer the frosting to a piping bag fitted with a large closed star tip (like Wilton 2D or Ateco 852). Hold the piping bag straight up, with the tip pointed down. Starting at the center of each cupcake pipe frosting into a rosette on top of each cupcake. (Start in the center of the cookie and continue to move around the circle of the cookie until you get close to the edges.) If you don’t have a piping tip, you can use an offset spatula to frost the cupcakes.
Serve & Store. Store 1-2 days at room temperature in an airtight container or refrigerate up to 3-4 days. Cupcakes can also be frozen up to 2 months.