Make Cookie Dough:
In a medium bowl, whisk together flour, baking powder, and salt.
In a separate large bowl, cream softened butter and granulated sugar until light and fluffy (2–3 minutes).
Beat in egg and vanilla. Gradually mix in dry ingredients until just combined.
Cover dough and chill for 30 minutes.

Prepare Cinnamon Filling:
Mix softened butter, brown sugar, and cinnamon in a small bowl until smooth and spreadable.

Roll and Fill Dough:
On a lightly floured surface, roll chilled dough into a rectangle about ¼-inch thick.
Spread cinnamon filling evenly, leaving a small border.
Roll dough tightly from the long edge into a log. Wrap in plastic and chill another 30 minutes.

Slice and Bake:
Preheat oven to 350°F (175°C). Line a baking sheet with parchment.
Slice chilled dough log into ¼-inch thick rounds and place 2 inches apart on the baking sheet.
Bake for 10–12 minutes, until edges are lightly golden. Cool on the sheet for 5 minutes, then transfer to a wire rack.

Glaze the Cookies:
In a small bowl, whisk powdered sugar, milk, and vanilla until smooth.
Drizzle over cooled cookies. Let glaze set before serving.