Double Chocolate Peanut Butter Sandwich Cookies

Ingredients

• 1/2 cup butter (room temperature)
• 1/2 cup white sugar
• 1/2 cup brown sugar
• 1 egg (room temperature)
• 1 tsp vanilla
• 1 cup flour
• 1/2 cup + 2 tbsp unsweetened cocoa powder
• 1 tsp baking soda
• 1/4 tsp salt
• 2 tbsp milk
• 1 cup mini chocolate chips (regular are fine, too)
• Candy pearls or sprinkles (optional)
• 1 cup smooth peanut butter
• 1/4 cup powdered sugar