Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large bowl, beat the softened butter and packed brown sugar until light and fluffy, about 2–3 minutes.
Add the egg and vanilla, mixing until glossy and combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry mix to the wet ingredients, stirring just until combined. The dough should be soft and cohesive.
Shaping the cookies
Lightly flour a work surface. Roll the dough to about 1/4-inch thickness.
Cut even rectangles, roughly Pop-Tart sized or smaller for snack cookies.
Place half the rectangles on the prepared sheet. Spoon about 1 teaspoon of packed brown sugar into the center of each rectangle.
Top with another rectangle and press the edges to seal. Use a fork to crimp if you’d like.
Baking
Bake for 12–15 minutes, until the edges are golden.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Glazing
Whisk powdered sugar and milk to make a smooth glaze.
Drizzle over cooled cookies and add sprinkles if desired. Let the glaze set before storing.
Notes
Store cooled cookies in an airtight container for up to 3 days. Freeze unglazed cookies for up to 2 months. Warm a cookie for 8–10 seconds in the microwave or 3–4 minutes at 300°F (150°C) for a refresh.