Preheat the oven to 350 degrees F. Grease the cavities of two mini muffin pans with nonstick cooking spray.

In a large bowl, add the butter and sugar. Beat using a hand mixer on medium speed until light and fluffy, about 2 minutes.

Add the eggs and vanilla to the mixture. Mix until the eggs are combined.

In a separate bowl, add the flour, cocoa powder, baking soda, and salt. Whisk until the mixture is well incorporated.

Add the dry ingredients to the wet ingredients and beat just until the dry ingredients are worked into the cookie dough. Scrape down the sides of the bowl and beat in any remaining dry ingredients.

Using a 1 tablespoon cookie scoop, scoop the dough and place in each cavity of the prepared mini muffin pans.

Bake the cookie cups for 7 to 8 minutes or until the cups are puffed in the center and set on the sides.

Remove the oven and let the cookies cool completely in the muffin pan. The centers will fall. Use a small medicine cup to make a deeper cup if you prefer.

Once the cookies are cool, turn them out onto a wire rack or paper towel.

In a large bowl, add the butter, powdered sugar, cocoa powder, vanilla extract, and salt. Beat using the hand mixer on medium speed until creamy.

If the frosting is too thick, add ½ tablespoon of milk to the mixture. Beat until light and fluffy, about 1 minute.

Place the frosting into a piping bag fitted with a large round tip.

Pipe the frosting into each cookie cup.

Refrigerate the cookies until the filling is set for a firm filling or leave at room temperature for a soft frosting filling.

Store in an airtight container at room temperature or in the refrigerator.