This ganache needs about an hour to thicken, so I recommend making it before you make whatever dessert you plan to fill or frost! I usually make it the night before. Heat 1 cup of heavy cream in a heatproof bowl in the microwave for 45 seconds, until it just begins to bubble and steam. If you don't have a microwave, you can also use a double boiler to heat the cream.
Gently pour 1 1/3 cups of dark chocolate chips into the heavy cream. Make sure the chips are fully covered with cream. Let the mixture sit for 1 minute. If you want to use milk chocolate to make the ganache, use 1/2 cup (120g) of heavy cream and 2 cups of milk chocolate (350g). Stir slowly until the cream and chocolate combine and the mixture is smooth. If any bits of chocolate aren't fully melted, heat the mixture again for 10 seconds and stir until smooth.
Then mix in 1/4 cup of room temperature unsalted butter, 2 Tbsp corn syrup, and 1/2 tsp fine salt until combined.
Cover with plastic wrap to prevent the ganache from forming a skin, then set aside to cool to room temperature. You can pour the mixture into a shallow, wide tray to help it cool faster if you're in a rush. Once it has thickened and reached room temperature, you can use it to fill or frost cakes, cupcakes, brownies, cheesecake, cookies, and even macarons! If you need to make this in advance, it can be covered and stored in the fridge for up to a month.