Preheat your oven to 350°F (175°C) to ensure even baking.
Cream together the softened butter and dark brown sugar in a large mixing bowl until light and fluffy.
Add the egg and vanilla extract to the mixture, stirring until well combined.
In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined; avoid overmixing.
Gently fold in the old-fashioned rolled oats, toasted pecans, white chocolate chunks, semi-sweet chocolate chunks, and dried tart cherries.
Drop spoonfuls of dough onto a parchment-lined cookie sheet, spacing them adequately for spreading.
Bake in the preheated oven for 10-12 minutes or until the edges are golden brown.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.