Preheat the oven to 350°F (175°C). Grease and line an 8x8-inch baking pan with parchment paper, letting the edges hang over for easy removal.

In a large bowl, cream the softened butter and sugar together until light and fluffy, about 2 to 3 minutes.

Beat in the egg and vanilla until fully combined.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually fold the dry mix into the butter mixture until it comes together into a soft dough.

Gently fold in 3/4 of the sprinkles, reserving the rest to press on top or for decoration.

Press the dough evenly into the prepared pan and sprinkle the remaining sprinkles evenly over the surface and press lightly so they stick.

Bake for 20–25 minutes, rotating the pan halfway through. The edges should be golden and the center set but still tender.

Remove from the oven and let cool completely in the pan on a wire rack.

Once cooled, lift the bars from the pan using the parchment overhang and slice into squares.

For a prettier finish, dust lightly with powdered sugar or drizzle melted white chocolate and sprinkle more jimmies on top.