Preheat oven to 350 degrees F. Grease six 2-1/2-inch muffin cups; set aside. In a medium bowl, whisk together egg, sugar, water, oil, baking powder, and vanilla. Add flour and cocoa powder, stirring until smooth. Spoon about 1 tablespoon batter into each muffin cup. Set a chilled peanut butter mound in center of chocolate batter in each muffin cup. Spoon remaining batter over peanut butter mounds.