I got this recipe from 365 days of Slow cooking. Its SOOO good!!
1 cup all-purpose flour 1 cup sugar (divided. this is important, don't just start dumping, read on!) 1 teaspoon baking powder 1/2 cup creamy peanut butter 1/2 cup milk 1 tablespoon canola oil 1 teaspoon vanilla extract 3 tablespoons unsweetened cocoa powder 1 cup boiling water Use a 4-quart slow cooker coated well with cooking spray. In a mixing bowl, combine flour, 1/2 cup sugar, baking powder.In the microwave, melt peanut butter for about 30 seconds, and then add to the mix along with milk, oil, and vanilla. Stir really well to combine.Spread this mixture into the bottom of your slow cooker.
In a separate bowl, mix together cocoa powder, the remaining 1/2 cup of sugar, and 1 cup boiling water (be careful!) Pour this evenly over the peanut butter batter in your cooker. Cover and cook on high for 2-4 hours, or until the top has set and an inserted toothpick (into the cake--underneath the cake is molten chocolate) comes out clean. Let the cake sit uncovered in an unplugged crockpot for about 20 minutes, then serve warm in dessert bowls.
I have a 6 qt crockpot so I set a glass over safe 4 qt dish inside my crockpot. It worked well.