Preheat your oven to 350°F. Prepare two mini-muffin pans by lightly spraying with cooking spray.
Beat the peanut butter, sugar, baking soda, and salt at medium speed of your mixer, until well-blended.
Add the egg and vanilla, and blend on low-medium speed until incorporated.
Scoop the dough by the tablespoonful into each cupcake well. Using a tart shaper or your hands, press the dough into the well along the bottom and along the sides.
Bake the cookies for 6 to 8 minutes. Remove them from the oven, and cool right in the cupcake pan. The cups may puff up in the oven, just simply press down the middles if they do.
For the ganache, put the chocolate chips in a bowl. Heat the milk and butter in a saucepan over low until hot but not boiling. Pour the heated milk and butter over the chocolate chips. Allow to sit for 5 minutes, then stir until smooth.
Once the cookie cups have cooled, fill each cup with chocolate ganache. Top with Cadbury mini eggs.