In a medium bowl use a mixer (or hand mixer) to beat butter and sugar. Start on a slow speed and work your way up. Beat until fluffy - about two minutes.
Scrape the bowl, add 1/4 cup of Baileys and vanilla.
Turn the beater to high and let it whip up for 4 minutes. You may need to pause and scrape the bowl a few times.
Taste the Baileys Buttercream Frosting and test the strength. If you want it a little more boozy add an extra two tablespoons. Whip for another few minutes.