In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large bowl, beat butter with brown sugar and granulated sugar until fluffy.

Add egg and vanilla, mixing until just combined. Gradually mix in dry ingredients until crumbly dough forms.

Divide dough into two disks, wrap in plastic, and refrigerate for at least 1 hour.

Mix filling ingredients in a small bowl, set aside.

Preheat oven to 350°F (175°C). Roll out dough to ¼ inch thick and cut into rectangles.

Place filling on half of the rectangles, top with remaining rectangles, and crimp edges to seal.

Brush tops with milk. Bake for 12-15 minutes until golden brown. Cool for 5 minutes before transferring to a wire rack.

Whisk powdered sugar and milk for icing, then drizzle over cooled cookies. Add sprinkles if desired.