Cookie Dough: Cream the butter and both sugars in a large bowl until fluffy. Add in the egg and vanilla extract, stir until well mixed.
Add in the flour, baking powder, salt, and cinnamon and mix until just combined. The dough will be very crumbly.
Divide the dough into 6 equal-sized portions and then form each portion into a ball. Press together until the dough comes together.
In a separate bowl, add ½ TBSP cinnamon and ¼ cup sugar to it and then roll each dough ball into it to coat the entire ball.
Place the dough onto a baking sheet and put it into the fridge to chill for at least 30 minutes before baking.
Bake: While the cookies are chilling, preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
Remove the dough from the fridge and place on your parchment paper lined baking sheet. Bake in the oven for 15-18 minutes or until the edges are just barely golden brown. Let the cookies cool for 5 minutes before transferring to a wire rack to cool completely.
Frosting: Whip the butter in a large bowl or the bowl of a stand mixer, until it’s fluffy. Add in the powdered sugar, one tablespoon of milk, and vanilla extract. Mix again to combine. Add the additional milk, if needed, to thin down the frosting if it is too thick.
Pipe the buttercream around the top of the cookies in a swirl pattern and add some additional cinnamon sugar on top for garnish. Serve and enjoy!