Bring the butter and milk or cream to room temperature. If your bowls are cold, run the outside of the bowls under some warm water and dry off before using.
Place the butter and vanilla into a large mixing bowl and start beating with a whisk attachment on a low to medium speed for 2-3 minutes. Then crank up the speed to high and beat for 10 minutes, scraping down the bowl in regular intervals to make sure all the butter is being beaten. The butter will become soft, fluffy, and almost white in color. *note 3
150 grams butter, 2 teaspoon vanilla extract
Sieve the icing sugar over the butter, add the salt, and using the paddle attachment and beat the icing sugar into the butter to form a thick frosting. The cake beaters of a handheld electric mixer will work just fine. *note 4
330 grams icing sugar, ½ teaspoon table salt
Add in the milk or cream 1 tablespoon at a time and beat on high to create a spreadable/pipeable frosting.
1-3 tablespoon cream/milk
With the paddle attachment, run at a low to medium speed for an additional 2-5 minutes. This will knock out the large air pockets in the frosting.
To make sure the buttercream frosting is smooth, use a silicon spatula to stir through the frosting a few times. This step is important especially if a hand-held electric mixer has been used.
If the icing or frosting feels too soft, cover it with plastic wrap or cling film and allow it to set at room temperature for 30 minutes to allow the whole mixture to cool down. The heat from the mixer and friction may make the frosting/icing feel a bit soft.
Transfer the frosting into a piping bag with a tip. For large tall swirls, begin the circle on the outer edge moving inwards and then on top of the swirl. For a single rosette, start in the middle of the cupcake and work your way to the edge. *note 5&6