In a medium bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, whisk milk, oil, egg, and vanilla until smooth.
Pour the wet mixture into the dry ingredients and stir until just blended to avoid toughness.
Gently fold in the rainbow sprinkles to keep colors bright.
Transfer the batter into the prepared pan and bake at 350°F for about 40 minutes, until a toothpick inserted into the center comes out clean and the edges crackle.
Glaze
In a small bowl, whisk powdered sugar with milk until silky.
After the loaf cools completely, drizzle the glaze on top and sprinkle extra sprinkles if desired.
Notes
During prep, sifting the flour twice gave the lightest crumb.While baking, tenting with foil at the 30-minute mark prevented the edges from over-browning.For storage, slice before freezing and layer with parchment to keep slices from sticking.