01 - Set oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large mixing bowl, whisk together flour, granulated sugar, baking powder, baking soda, and salt until fully blended.
03 - In a separate bowl, whisk together melted butter, lemon-lime soda, eggs, vanilla extract, and sour cream or Greek yogurt until smooth.
04 - Pour wet mixture into dry ingredients. Gently fold together using a spatula until just combined.
05 - Fold in rainbow sprinkles and salted pretzel pieces, taking care not to overmix.
06 - Evenly distribute batter into prepared muffin tin, filling each cup about three-quarters full.
07 - In a small bowl, mix melted butter, granulated sugar, and ground cinnamon. Drizzle or brush over the tops of batter in each muffin cup. Sprinkle extra rainbow sprinkles if desired.
08 - Bake muffins for 22 to 25 minutes, or until a toothpick inserted in the center comes out clean.
09 - Allow muffins to cool in the pan for 5 minutes. Transfer to a wire rack and cool completely.