For the cupcakes:
Preheat oven to 350 degrees F.

In a bowl, whisk egg and sugar until smooth and no lumps remain. Add milk, cream, butter and vanilla, and mix until combined. Stir in sour cream.

Sift dry ingredients together and then add to the wet mixture. Mix until batter is smooth, then swirl in fudge sauce. Line a muffin tin (12 count) with liners and using a 1/4 cup measure, add batter to each cup. Bake for 15-18 minutes. Let cool before frosting.

For the frosting:
Cream butter and peanut butter together in the bowl of an electric mixer. Add powdered sugar with mixer on low speed and gradually add milk and vanilla extract. Add a pinch of salt and mix until frosting is smooth. If too thick, add more milk; if too thin, add more sugar.

Put the frosting in a pastry bag fitted with a tip. Poke a hole in the cupcake with the pastry tip and fill the cupcake. Then frost the cooled cupcakes as desired.

For the homemade fudge:
Combine sugar, cocoa, salt, and flour in a bowl or pot over a double boiler. Bring water or milk to a boil in another saucepan or in the microwave. Gradually add hot liquid to sugar mixture. Stir constantly until mixture thickens.

Once it has reached the desired thickness, remove from heat and stir in butter and vanilla. Drizzle over cupcakes and store the rest of the sauce in the fridge for 1-2 weeks.