In a medium bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt until evenly combined.
In a separate bowl, stir the milk, peanut butter, vegetable oil, and vanilla extract until the peanut butter is mostly smooth.
Pour the wet mixture into the dry ingredients and stir just until a thick batter forms with no large streaks of flour remaining.
Lightly grease the insert of a slow cooker and spread the batter evenly across the bottom.
Sprinkle the peanut butter chips over the surface of the batter in an even layer.
Carefully pour the boiling water over the top without stirring. Cover and cook on high for 2 hours until the cake looks set around the edges but still has a soft, saucy center.