Line a slow cooker with parchment or grease well. Melt butter and stir in chocolate chips until melted. Combine melted chocolate and the rest of the Cake Batter ingredients in a large bowl, and stir until just combined, then pour into the prepared slow cooker. Mix together Liquid Layer ingredients, then pour over batter layer but do not mix together.
Cover and cook on high until set, about 2 hours.
Allow to cool slightly before serving. Top with whipped cream or ice cream.