01 - In a large heavy-bottomed saucepan, combine chopped apples, brown sugar, apple cider, cinnamon, nutmeg, cloves, allspice, vanilla extract, and salt; stir well.
02 - Cook over medium heat, stirring occasionally, until apples become very soft, approximately 30 minutes.
03 - Use an immersion blender to purée the mixture until smooth, or blend in batches using a countertop blender and return to the pot.
04 - Reduce heat to low and simmer uncovered, stirring frequently, until the mixture thickens and darkens into a spreadable butter, about 45 minutes; stir often to prevent sticking.
05 - Taste and adjust sweetness or spices as desired, then allow to cool slightly.
06 - Toast the bread slices to desired crispness and optionally spread with unsalted butter.
07 - Generously spread the warm spiced apple butter over the toasted bread and serve immediately.