Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the rolled oats, dark brown sugar, and cinnamon until evenly coated. Spread this mixture onto a baking sheet lined with parchment paper. Bake for 10-12 minutes, or until the oats are lightly toasted and fragrant. Remove from the oven and let it cool for a few minutes.

In a stand mixer, beat together the softened butter and remaining dark brown sugar on medium speed until creamy, about 2-3 minutes. Add the eggs one at a time, mixing well after each addition. Gradually incorporate the cooled oat mixture along with any remaining dry ingredients until fully blended, creating a sticky cookie dough. Delicately fold in the chopped dried prunes for a delicious twist.

Cover the mixing bowl with plastic wrap and refrigerate the cookie dough for at least 1 hour, or overnight for enhanced flavor and texture.

After chilling, preheat your oven again to 350°F (175°C) if it's not already warm. Line baking sheets with parchment paper. Using a cookie scoop or tablespoon, place rounded portions of dough onto the prepared sheets, making sure to leave space between each cookie as they will spread while baking.

Slide the cookie sheets into the preheated oven and bake for 10-12 minutes, or until the edges are lightly golden but the centers still appear soft. Once baked, remove them from the oven and allow the cookies to cool on the sheets for a few minutes before transferring them to a wire rack.