Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, cream together the butter and brown sugar until light and fluffy.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the flour, baking powder, and cinnamon.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the flour mixture. Mix until just combined.
Drop spoonfuls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 15-20 minutes or until golden brown around the edges.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.
Notes
Store cookies in an airtight container for up to 5 days. Reheat slightly before serving for best texture.