01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving a slight overhang for easy removal.
02 - In a large mixing bowl, beat the softened butter, granulated sugar, and light brown sugar with an electric mixer until pale and fluffy, about 2 to 3 minutes.
03 - Incorporate eggs and vanilla extract into the mixture, beating until fully blended.
04 - In a separate bowl, whisk together the all-purpose flour, baking powder, and salt.
05 - Gradually stir the dry mixture into the wet ingredients, mixing just until no flour pockets remain.
06 - Gently fold in the rainbow sprinkles using a rubber spatula, ensuring even distribution without overmixing.
07 - Spread the dough evenly in the prepared pan and smooth the surface with a spatula.
08 - In a small bowl, stir together granulated sugar and ground cinnamon for the topping.
09 - Sprinkle cinnamon sugar evenly over the batter. Bake for 28 to 32 minutes, until edges are golden and a toothpick inserted in the center comes out clean.
10 - Let bars cool completely in the pan set on a wire rack. Lift out using the parchment overhang and slice into bars.