Line slow cooker with heavy duty foil and spray with vegetable oil spray.

Whisk flour, 1/2 cup sugar, 1/4 cup cocoa, baking powder, and salt together in a large bowl. In a separate bowl, whisk together milk, butter, egg yolk, and vanilla. Stir milk mixture into flour mixture until just combined. Fold in chocolate chiops (batter will be stiff). Scrape batter into prepared slow cooker and spread to edges.

Mix remaining 1/2 cup sugar with remaining 1/4 cup cocoa, then sprinkle over top. Slowly pour boiling water over top. Do not stir.

Cover and cook until top of cake looks cracked, sauce is bubbling and toothpick inserted into cakey area comes out with moist crumbs attached, about 1 1/2 hours on high.

Let sit for 10 minutes before serving. Spoon over vanilla ice cream.