Add all ingredients to a 4-quart slow cooker. (You can use a 6 quart, it'll just heat faster so keep an eye out.) Cover and cook on low for 2 hours, stirring roughly every 30 minutes.
Line a baking sheet with parchment paper and pour in the batter. We'd recommend either a 9x9 or 9x13, depending on how thick you'd like it.
Chill in the refrigerator at least 4 hours (but ideally overnight) before cutting.
Notes
Try stirring in 3/4 cup of peanut butter, or 3/4 cup of your favorite nuts or candy for a fun twist. Or if you're feeling really decadent, swirl in some dulce de leche before refrigerating.