Step 1: Preheat the Oven. Begin by preheating your oven to 350°F (175°C).
Step 2: Prepare Your Baking Pan. Line a 9x13 inch baking pan with parchment paper and grease it lightly.
Step 3: Combine Dry Ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Step 4: Cream Butter and Sugar. Beat the butter and sugar until light and fluffy, about 2-3 minutes.
Step 5: Add Egg and Vanilla. Mix in the egg and vanilla extract until fully combined.
Step 6: Mix in Dry Ingredients. Gradually add the dry mixture until just combined.
Step 7: Spread the Batter in the Pan. Pour the batter into the prepared pan and spread it evenly.
Step 8: Bake. Bake for 20-25 minutes, until edges are golden and a toothpick comes out clean.
Step 9: Cool Completely. Allow to cool in the pan for 10 minutes, then transfer to a wire rack.
Step 10: Make the Frosting. Beat cream cheese and remaining butter until creamy, then add powdered sugar and vanilla.
Step 11: Frost the Bars. Spread the frosting evenly over the cooled bars.
Step 12: Decorate. Sprinkle with rainbow sprinkles.
Step 13: Slice and Serve. Cut into squares and enjoy!