01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy lifting.
02 - In a medium bowl, whisk together melted butter and brown sugar until the mixture appears smooth. Incorporate the egg and vanilla extract, whisking until fully blended.
03 - Stir in the lemon-lime soda until just combined.
04 - In a separate bowl, whisk flour, baking powder, and salt. Gently fold these dry ingredients into the wet mixture until no streaks remain, avoiding overmixing.
05 - Add the rainbow sprinkles and fold carefully to distribute evenly.
06 - Spread the blondie mixture evenly into the prepared pan.
07 - In a small bowl, combine granulated sugar, flour, melted butter, lemon-lime soda, and a pinch of salt. Mix with a fork until crumbly. Stir in the rainbow sprinkles.
08 - Evenly sprinkle the streusel mixture over the blondie batter.
09 - Bake at 350°F for 26 to 28 minutes, or until the edges are set and a toothpick inserted into the center results in a few moist crumbs.
10 - Cool completely in the pan on a wire rack. Lift out using parchment overhang and slice into 16 bars.