Peel, core and cut apples into chunks. In a large Dutch oven set over medium heat, add apples, butter, brown sugar, cinnamon and salt. Simmer, stirring occasionally, for 25 to 30 minutes or until apples are tender.
Using an immersion blender, blend apples until smooth.
Transfer mixture to slow cooker. Cook for 4 to 6 hours on high, stirring every hour, or until deep amber. Cool completely.
Transfer to airtight containers, store refrigerated for up to 5 days.
To process apple butter: Ladle hot apple butter into a hot, sanitized jar to within 1/4-inch (0.5 cm) of top of jar. Insert a non-metallic utensil to remove any air bubbles. Add more apple butter to raise level and maintain a 1/4-inch (0.5 cm) headspace. Wipe the jar rims with a clean paper towel to remove any food residue. Center the hot lids on the clean rims and screw on the bands until resistance is met, then increase to fingertip tight. Repeat for remaining apple butter.
Add the filled jars to the canner and ensure the jars are covered with at least 1-inch (2.5 cm) of water. Cover the canner and bring to a full boil. Process the jars for 10 minutes, starting the time from when the water returns to a full boil.
After processing, remove the lid from the canner, wait 5 minutes and carefully lift the jars. Place them on a cooling rack or a flat surface lined with a towel. Cool undisturbed for 24 hours. Do not retighten the lids. While the jars cool you will hear popping sounds and the lids should curve downwards. This indicates that the seal has formed. If the jars fail to seal, refrigerate and consume within 1 week.
Label and store in a cool dark place. Use within 1 year.
Tip for freezing: Cool completely. Transfer to freezer safe containers. Freeze for up to 1 month.
Tip for canning: To sanitize jars: Place 4 clean (250 mL) jars and screw bands in a boiling water canner fitted with a metal rack. Cover with water. Bring to a boil; boil for 10 minutes. Remove from heat and let stand in hot water until needed. Bring a saucepan of water to a boil; remove from heat. Place seal lids in saucepan, let stand in hot water for at least 5 minutes or until ready to use. Jars, bands and lids must be kept hot until ready to fill.