How to thicken buttercream without adding more sugar
It looks like a couple of similar recipes in your link there, but you could make it a teensy bit thicker without increasing the sweetness by 1) diminishing slightly the amount of liqueur, or 2) adding a little bit of unsweetened cocoa powder [but of course it will then become brown and chocolatey], or 3) adding a little more cornstarch [since confectioners sugar does contain cornstarch anyway]... or some combination of above.  Go easy at first of course.