Preheat your oven to 375°F (190°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 2-3 minutes. Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry mixture to the wet mixture, mixing on low speed until just combined. Do not overmix.
Press the dough evenly into the prepared pan. Bake for 20-25 minutes, or until the edges are lightly golden and the center is set. Let cool completely in the pan on a wire rack.
While the bars cool, make the frosting: Beat the softened butter until creamy. Gradually add the powdered sugar, then the heavy cream and vanilla, beating until smooth and spreadable.
Once the cookie base is completely cool, spread the frosting evenly over the top. Immediately sprinkle with red, white, and blue sprinkles. Use the parchment to lift the bars out, then cut into squares or rectangles.