3 cups semi sweet chocolate chips
1 (14-ounce) can sweetened condensed milk
1 teaspoon pure vanilla extract
1 tablespoon salted butter
cooking spray to spray down the crock
parchment paper or non-stick foil to line a 9 x 9 baking pan
Use a 4-quart slow cooker. If you only have a 6-quart, that's okay, but it will melt faster, so keep an eye on your chocolate.
Spray the crock well with cooking spray (if you are using a Ninja, I'd still spray it down).
Dump everything into the pot. Cover, and cook on high for about 1 hour, or on low for about 2 hours. Stir every 30 minutes. Once the chocolate has lost it's form and can be stirred into a liquid state, spread it out into a lined 9 x 9 baking pan.
Chill in the refrigerator overnight (or at least 4 hours) before cutting into bite-sized squares.
If you'd like to, go ahead and add chopped nuts or candy canes for a peppermint version.
Note: This recipe is from A Year of Slow Cooking.