In a medium bowl, whisk together the flour, baking powder, and salt. In a separate large bowl, cream together the softened butter and brown sugar until light and fluffy—about 3-5 minutes. Add egg and vanilla extract; mix until smooth.

Gradually incorporate the flour mixture into the wet ingredients. The dough will become slightly sticky but manageable.

In another bowl, combine brown sugar, cornstarch, cinnamon, and pinch of salt for the filling.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Roll the dough into small balls, flatten into discs, spoon the filling onto half the discs, place another disc on top, sealing the edges.

Bake for 12-15 minutes, or until the edges are golden brown. Allow cookies to cool completely on a wire rack.

While the cookies cool, whisk together powdered sugar, milk, and vanilla until smooth. Drizzle or spread icing over cooled cookies and garnish with sprinkles, if desired.