Your butter should be at room temperature that's the most important thing of this recipe to be done perfectly. Secondly the sugar should be in a fine powder and it is sifted 4-5 times through a fine sieve. Every ingredients must be on a room temperature and we need an electric hand mixer or a stand mixer for this recipe. Also measurement is very important. If you like more sweet than add 5 cups of sugar else 4 1/2 is perfect anyhow it's upto you.

To start with in a big bowl add the unsalted butter at room temperature which is a must then start beating it with the electric hand mixer starting from medium speed then to high and beet till the butter got stiff peaks it takes around 12-15 minutes. Then add the 4-5 times sifted powdered sugar 1 or 2 tbsp at a time. Don't add another spoon of sugar until the first one completely mix in it. Now this work need patience but when you will do like this it's end result will be amazing.

And allow it to come on room temperature naturally don't heat it. Follow all the steps and make a perfect buttercream ftisting all the time.
Happy baking