01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving a slight overhang for easy removal.
02 - In a large bowl, cream together the butter and sugar with an electric mixer until light and fluffy, about 2-3 minutes.
03 - Add the egg and vanilla extract. Beat until fully combined.
04 - In a separate bowl, whisk together the flour, baking powder, and salt until well blended.
05 - Gradually add the dry ingredients to the wet mixture, mixing until just combined. Do not overmix.
06 - Press the dough evenly into the prepared pan using clean hands or a spatula.
07 - Bake for 18-22 minutes, or until the edges are lightly golden and the center is set. Do not overbake.
08 - Let the bars cool completely in the pan on a wire rack.
09 - Beat the butter until creamy. Gradually add powdered sugar, then milk, vanilla, and salt. Beat until smooth and fluffy. Adjust milk if needed for spreadable consistency. Add food coloring if desired.
10 - Spread the frosting evenly over the cooled bars. Top with sprinkles if using.
11 - Lift the bars out of the pan using the parchment overhang. Slice into 24 squares.