My entire home always smells amazing when this is being slow cooked! Tastes even better than it smells

In a medium mixing bowl, whisk together flour, granulated sugar, cocoa powder, baking powder, and salt.
In a separate bowl, blend the milk, vegetable oil, and vanilla extract. Add this to the dry ingredients and mix until just combined.

Stir in the smooth peanut butter until the mixture is smooth and well incorporated.
Spread the batter evenly in the bottom of a greased slow cooker.
Sprinkle the chocolate chips evenly over the top of the batter.
In a small bowl, mix the brown sugar with the boiling water, stirring until the sugar is dissolved. Carefully pour this mixture over the contents of the slow cooker—do not stir.
Place the lid on the slow cooker and set it to cook on high for 2 to 2.5 hours, or until the top is set and slightly puffed.

Turn off the slow cooker and allow the cake to rest for 30 minutes with the lid off before serving. This will let the cake firm up a bit and allow the sauce to thicken.
Variations & Tips
For those with a penchant for extra peanut flavor, swirl an additional quarter cup of peanut butter into the batter before cooking.
Individuals with a fondness for darker chocolate can substitute a portion of the semisweet chocolate chips for dark chocolate chips or chunks.
Keep in mind that slow cookers can vary in temperature and power, so the first time you make this cake, watch the cooking time closely to avoid overcooking.