In a large bowl, stir together the peanut butter, oats, icing sugar, and vanilla until completely combined. You may need to use your hands towards the end to gather everything together.
Shape the mixture into 2-inch (5cm) wide, 1/2-inch (1cm) thick circles. You should get about 10 -12 cookies. I pressed mine into a cookie cutter to get perfect circles.
Carefully place the cookies onto a baking tray lined with parchment paper, or silicone mat, and place in the freezer until firm - about 20 - 30 minutes.
Drizzle the cookies with melted chocolate, if desired.
Notes
Cookies should be kept refrigerated in an airtight container. They can also be frozen for up to 1 month. Thaw overnight in the fridge.