Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until well combined.

Cream the softened unsalted butter and granulated sugar using an electric mixer on medium speed for 2-3 minutes until light and fluffy.

Beat in the egg and vanilla extract until smooth and combined.

Gradually add the dry mixture into the wet ingredients, stirring gently until just combined.

In a small bowl, combine the brown sugar and ground cinnamon until evenly mixed.

Roll out the cookie dough into a rectangle about ¼ inch thick on a floured surface.

Sprinkle the cinnamon filling evenly over the rolled-out dough.

Roll the dough from the longest edge into a tight log and wrap in plastic wrap. Chill in the refrigerator for 30 minutes.

Slice the log into ¼ inch thick rounds and place them on the prepared baking sheets.

Bake for 10-12 minutes until the edges are lightly golden. Keep an eye to avoid overbaking.

Cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

Whisk together the confectioners' sugar, milk, and vanilla extract until smooth. Adjust milk for desired consistency.

Drizzle the icing over completely cooled cookies and let it set before serving.