This is my go to buttercream frosting, you can add some variations to it but it is always my starting  point. Try adding some lemon rind or almond extract for some subtle flavour. It is the perfect piping and filling consistency. You an also easily adjust the colour to what you need by adding food colouring.

Ingredients:

½ Cup unsalted butter (softened)
4 cups powdered sugar
1 tablespoon cream
1 teaspoon vanilla
1 pinch salt 

Directions:

In a bowl, using an electric mixer, or a stand mixer with the paddle attachment,  mix your butter on medium speed for about 1 to 2 minutes until smooth. Having your butter at room temperature makes this easiest, if you find your frosting is clumpy at the end it is because your butter was not smooth at this stage. 

Using a sifter, add 2 cups of the powdered sugar, one at a time and continue whisking the mixture. Add a pinch of salt, your vanilla, and cream and whisk until well combined. Add the remaining powdered sugar, one cup at a time, while continuing to whisk. Once it is combined, turn your mixer to medium high and mix for an additional 8 minutes. 

You can change the consistency of your buttercream frosting by adding more sugar (for thicker frosting) or more cream (for looser frosting) or omitting the cream all together.