1. Cream together the butter and sugars until fluffy. Add in the egg and Irish cream and mix until well combined.
2. In a separate bowl combine flour, salt and baking soda. Add flour mixture to wet ingredients and stir together. Form into a ball, wrap with plastic wrap and refrigerate for at least an hour.
3. Preheat the oven to 350°F (175°C).
4. Using a cookie scoop, or a spoon, drop evenly sized balls of cookie dough onto ungreased cookie sheets. Flatten cookie balls slightly (as these cookies don't spread much). If you aren't going to put frosting on the cookies, you can put sprinkles on the dough at this stage before baking.
5. Bake in preheated oven until the cookies are golden brown, 12-15 minutes. Cool completely before frosting.

Frosting

1. Using a mixer, whip the butter for about a minute until it is fluffy, then gradually add in the powdered sugar until it comes together as frosting. Add in the Irish cream and whip for another minute, until it is well incorporated.
2. Once cookies are completely cooled, spread frosting evenly on each cookie. Top with sprinkles if desired. Serve immediately, or store covered in the fridge.