Preheat the oven to 375℉. Lightly spray a mini muffin pan with nonstick cooking spray.
In a stand mixer or a large bowl with a hand mixer, cream together butter, peanut butter, and both sugars. Mix in egg and vanilla. Add flour, baking soda, and salt to the mixing bowl and mix until combined.
Press the dough into the bottom and up the sides, then make an indentation in the center using a small lid, the back of a teaspoon, or the end of a wooden spoon handle. For the best shape, indent the cookie cups both before and after baking to help the centers stay defined.
Bake the cookie cups for 11-14 minutes, until the edges are set. Carefully press the centers back down again while they're still warm.
Whipped Peanut Butter Frosting
Add butter and peanut butter to a stand mixer with a whisk attachment and whip for about 5 minutes, until it becomes very light tan and fluffy. It should lose its glossy shine and hold stiff peaks.
Add powdered sugar and 2-3 tablespoons of milk until you reach the desired consistency.
Use an icing bag with a Wilton 2A circle tip to add frosting into each cookie cup.
Chocolate Ganache
Heat heavy whipping cream in a small dish in the microwave until steaming, but not simmering. Add chocolate to the dish and stir until it melts.
Drizzle chocolate ganache over each cookie cup, then let it set.