Place the butter and pecan nut butter into a medium bowl and beat with an electric mixer.
Gradually mix in the sugar and cocoa.
When the frosting starts to get thick, incorporate milk and bourbon one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable.
Beat for at least 3 minutes for it to get good and fluffy.
Notes
The color variety in the frostings in the photo comes from using different varieties of chocolate and more or less milk.