In a stand mixer, cream the butter with a paddle attachment.
Scrap down sides and add powdered sugar a cup at a time, mixing between cups and scraping down.
Once all of the sugar is added, beat on medium-high speed for 2 to3 minutes.
Add cocoa powder, chocolate liqueur, vanilla extract, and salt and beat in low until full incorporated.
Keep mixer going on low speed while slowly pouring in the heavy cream.
Once heavy cream is fully incorporated, turn off and scrape down the sides of the bowl.
Turn mixer on medium-high speed and beat for 3 minutes until buttercream is light and fluffy.