FILLING: Combine all filling ingredients in a small bowl and beat till smooth. Cover and chill for about 45 minutes.
Measure out about a teaspoon of filling and roll it into a ball. Repeat with the remaining peanut butter mixture. Place the peanut butter balls in the freezer for at least 30 minutes.
CUPCAKES: Whisk together flour, sugar, cocoa, soda, and salt in a large mixing bowl. Add oil, water, and vinegar. Whisk till smooth.
Fill paper lined muffin cups just over halfway full. Press a frozen peanut butter ball in the center till it is covered with batter.
Bake at 375° for about 18-20 minutes.
FROSTING: Beat butter, peanut butter, vanilla, and salt in a large bowl. Add 5 cups powdered sugar and 6 tablespoons of milk. Beat till smooth. Add additional powdered sugar or milk to get the consistency you need.