• In a mixing bowl, combine the softened butter and sugar.
• Cream the butter-sugar mixture over medium speed and wait until it turns fluffy and pale in color. This process will take about 3-4 minutes.
• Add the egg and the vanilla extract. Mix until well combined.
• Slowly incorporate the dry ingredients and mix thoroughly. Your cookie dough will be thick.
• Meanwhile, grease a mini muffin tin.
• To make balls, scoop about 2 large tsp and press them into the bottom muffin cup. Slightly press the center to make an indention, where you'll place the Reese's later.
• Bake these cookies in at preheated oven of 350 degrees F for 8 minutes. While baking, prepare all the Reese's.
• Remove cookies from the oven (the centers may look a little too soft but don't worry, they will firm more as they cool.
• Add the Reese's into the center of each of the cookie cups.
• Allow cooling for 3-4 minutes in the muffin tin.
• Remove cookies and transfer them to a cooling rack to finish cooling.

To make the buttercream:

• Combine the shortening and butter in a large bowl and cream together until smooth.
• Add the peanut butter and mix until well combined.
• Slowly add the powdered sugar while continuously mixing.
• To thin out the buttercream, you can add some milk or water, as needed, and mix until well combined and smooth.
• Transfer buttercream into a piping bag with a piping tip and use it to decorate your cookie cups.