Start With 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 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. 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.)
Troubleshooting. If the frosting is too thick or stiff to pipe or spread easily, add more cream 1 teaspoon at a time, until you reach your desired texture. If the frosting is too thin, add more powdered sugar or cocoa powder 1 Tablespoon at a time until you reach the correct texture. If the frosting warms up too much and starts to melt, chill in the refrigerator for 15-20 minutes or until it's set again.
Spread or pipe frosting on cake as desired. Chill cake afterward to keep fresh.
Storage & Make Ahead Tips. Frosting can be made in advance and chilled in an airtight container in the fridge up to 2 weeks or frozen up to 1 month. Thaw frozen frosting in the refrigerator and bring chilled frosting to room temperature and mix thoroughly before using.