In a large bowl or glass measuring cup whisk the oil, butter, eggs, and vanilla.
Add the peanut butter and whisk until smooth and incorporated.
Fold in the dry ingredients until no dry streaks remain.
Fold in candy, reserving ½ cup for topping.
Prepare a 4-6 quart slow cooker by spraying cooker with baking spray. Press in one 12x15 inch piece of parchment paper, creasing edges so it can lay flat. Spray parchment paper.
Pour in prepared batter and top with lid. Cook on high 3 ½ to 4 hours. The cake will be set in the middle and a toothpick will come out clean.
In the last 30 minutes of cooking, top with reserved candy and finish cooking.