In a medium bowl, stir together the peanut butter and egg until smooth and creamy. Stir in the vanilla, cinnamon, and salt until fully incorporated. Stir in the maple syrup. Stir in the milk. Pour the oats and flour into the bowl, and sprinkle the baking powder evenly over the top of the oats and flour (to prevent clumping!). Stir all three ingredients in together until just incorporated. Chill the cookie dough for 20 minutes.
Preheat the oven to 325°F, and line a baking sheet with a silicone baking mat or parchment paper.
Using a spoon and spatula, drop the cookie dough into 15 rounded scoops onto the prepared sheet. Flatten to 3/8″-thick using a spatula. Bake at 325°F for 8-10 minutes. Cool on the pan for 10 minutes before turning out onto a wire rack.