Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or lightly grease it.
In a mixing bowl, cream together the softened butter and brown sugar until light and fluffy.
Add the egg and vanilla extract and mix until fully combined.
In a separate bowl, whisk together the flour, baking powder, salt, and cinnamon. Gradually add the dry ingredients to the wet mixture, mixing just until combined.
Spread the dough evenly into the prepared pan.
In a small bowl, mix together the brown sugar, cinnamon, and melted butter for the swirl. Drop spoonfuls over the dough, then use a knife or toothpick to gently swirl it through the batter.
Bake for 22–25 minutes, or until the edges are golden and a toothpick inserted into the center comes out mostly clean (a few moist crumbs are okay).
Let cool for 10–15 minutes. While the bars are cooling, whisk together the powdered sugar, milk, and vanilla extract to make the glaze.
Drizzle the glaze over the cooled bars. Let it set before slicing into squares and serving.