Place the oven rack in the middle position and heat the oven to 350°F. Line a rimmed sheet pan with parchment paper.
Whisk oats, flour, baking powder, salt, and cinnamon in a medium bowl.
Whisk brown sugar and granulated sugar together in a medium bowl.
Whisk in melted butter until well combined.
Whisk in egg yolk, vanilla, and milk until smooth.
Use a rubber spatula to gently stir in the oat mixture until a soft cookie dough forms.
Gently fold in the plumped raisins.
Working with heaping 1 ½ tablespoons of dough at a time, roll into balls.
Evenly place the rolls on the prepared baking sheet.
Use the bottom of a greased drinking or measuring cup to flatten the dough balls to ½ inch thickness.
Bake the cookies until the edges are set and just beginning to brown, 13-15 minutes. Rotate the baking sheet halfway through baking.
Let the cookies cool on the baking sheet for 5 minutes.
Transfer the cookies to a wire rack to cool. Serve warm or at room temperature.