Preheat oven to 375ºF (190ºC) and prepare parchment-lined or lightly buttered baking sheets.
In a large bowl, cream together butter and sugar until smooth. Whisk in egg, milk and vanilla.
Add flour, baking powder, baking soda, cinnamon and salt and mix until combined. Add oats and raisins and continue mixing until well incorporated.
Refrigerate the cookie dough for 30-60 minutes.
Drop by heaping tablespoons, 5 inches apart, on prepared baking sheets and flatten them slightly.
Bake for 10-12 minutes until golden brown around the edges. The cookies might look under-baked, but they will continue to set as they cool. This is the secret to a soft and chewy oatmeal raisin cookie! Remove from the oven and let cool on baking sheets for 5 minutes where they will continue to finish baking. Transfer to a wire cooling rack to let cool completely.