Grease your crockpot with cooking spray or butter to prevent sticking.
In a large mixing bowl, combine 2 cups of all-purpose flour, 1 ½ cups of granulated sugar, ½ teaspoon of baking powder, ¼ teaspoon of salt, and 1 tablespoon of unsweetened cocoa powder.
In a separate bowl, whisk together ¾ cup of whole milk, ½ cup of vegetable oil, 2 large eggs, and 1 teaspoon of vanilla extract.
Gradually pour the wet ingredients into the dry ingredients, mixing until just combined. Avoid overmixing.
Fold in 8-10 Oreo cookies broken into pieces.
Chop or melt 1 candy bar (6 oz) and mix into the batter.
Pour the batter into the greased crockpot, spreading it evenly.
Cover the crockpot and cook on high for 2 ½ to 3 hours, or until a toothpick inserted into the center comes out clean.
Let the cake rest for 10-15 minutes before serving to set.
Scoop out individual portions and serve warm with a scoop of ice cream or whipped cream, if desired.