Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, and kosher salt.
In a large bowl, beat the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, then mix in the vanilla extract.
Gradually add the dry ingredients, mixing until just combined.
Fold the rainbow jimmies into the dough gently with a rubber spatula.
Scoop about 2 tablespoons of dough for each cookie and place them 2 inches apart on the prepared baking sheets.
Press a few extra sprinkles onto the tops of the dough balls, if desired.
Bake for 9 to 11 minutes, until the edges are lightly golden while the centers remain soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a cooling rack.
Drizzle with melted white chocolate if desired after completely cooling.
