Preheat oven to 350°F. Spray an 8-inch cake pan with baking spray.
Whisk together flour, cocoa powder, cinnamon, sugar, baking powder, and salt. Make a well in the center of the dry ingredients and add milk, butter, vanilla, and peanut butter. Stir until ingredients are well blended. Fold in chocolate chips. Spread batter in pan.
To make the topping, stir together cocoa powder and brown sugar. Sprinkle evenly over batter in pan. Pour boiling water over the top of the batter and DO NOT MIX. Bake 35-40 minutes, or until top looks crackled and center is almost set. Remove from oven and cool for 15-20 minutes. Serve warm.