Try sprinkling some baby powder or cornstarch on the spot, working it in a bit with a clean toothbrush, then letting it sit while the powder/cornstarch soaks up the butter. Brush off and repeat if necessary.
The first step is to clear the stain of any excess grease or oil with a clean, dry cloth. Never use water to clean such stains. It merely spreads the stain or embeds it deeper in the material. Finally, use a quality degreaser, such as one available from LeatherWorldTech.com. After using the degreaser, clean the area with Nubuck cleaner and then apply NuBuck protector. Use a NuBuck sponge/brush to raise the nap of the leather.
White vinegar applied with damp cloth, rinse then brush the area to bring back the nap. Test first of less visible part of the shoe.