Mix 1 stick softened butter, 1/4 cup light brown sugar and 1/2 teaspoon each ground nutmeg, cinnamon and cloves until combined. Put the butter on a piece of plastic wrap and roll into a 1-inch-thick log; wrap and chill 1 hour. Pour 3 cups apple cider into a slow cooker; cover and heat on high for 1 hour. Divide 1 cup bourbon among 6 glasses. Top each with hot cider and a slice of the spiced butter.
If you don't have a slow cooker, keep these drinks warm in a saucepan on the stovetop over low heat.