Make the Cake: Whisk flour, sugar, baking powder, salt, and cocoa powder in a large bowl. Add oil, vanilla, and milk and whisk until smooth.
Spray the insert of a 7-Qt slow cooker with nonstick cooking spray. Pour cake batter into insert.
Make the topping: add the cocoa, salt, and sugar to the boiling water and whisk to combine. Slowly pour over the cake batter in the crockpot insert.
Cover the insert with paper towels, place in crockpot and place lid on top of paper towels.
Set time for HIGH for 2 hours. Check cake after 1 hour and then again at 1.5 hours. Cake will look wet in center but the edges will look done and a toothpick will come out clean near the edge when done. All slow cookers cook differently, mine took 1.5 hours.
Serve warm with ice cream or whipped cream.
Notes
For easy clean up, use a slow cooker liner.
You can use regular or dutch process cocoa powder.
You can use any kind of milk.
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